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12 Evidence Project Final Report

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Project identification

1. Defra Project code PS2237

2. Project title

Development of a home and garden scenario for HardSPEC

3. Contractororganisation(s)

FERASand HuttonYorkYO41 1LZ

54. Total Defra project costs £ 18,975(agreed fixed price)

5. Project: start date................. 01/12/2008

EVID4 Evidence Project Final Report (Rev. 06/11) of 24

mailto:[email protected]

end date.................. 31/08/2009


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Executive Summary7. The executive summary must not exceed 2 sides in total of A4 and should be understandable to the intelligent

non-scientist. It should cover the main objectives, methods and findings of the research, together with any other significant events and options for new work.

The aim of this study was to provide supporting data to underpin parameters for, and to develop a home and garden scenario for HardSPEC. The objectives of the study were to:Define typical ratios of hard surface to non-hard surface in gardens.Establish probable quantity of dilution water.Estimate the number of residences served by the same drain that will be sprayed at one time.Develop a home & garden scenario for HardSPEC.Compare the output of the home & garden scenario to the current version of HardSPEC, and other data.

Five data sources were found that contained details on the size of front and rear gardens, and the percentage of areas that were paved. One of these data sources contained details of the hard surface materials. Despite being undertaken in different years, in different cities and using different techniques, the overall findings were broadly similar in that garden sizes decreased in the order: detached > semi-detached > terrace; the majority of houses have gardens (at least 98%, excluding flats); > 90% of back gardens have a patio; gardens represent 20 – 30% of the total land area in cities. All the data sources noted that paving over of front gardens was a serious concern. These data enabled an average size of garden and paving type in the garden to be determined.

Mastermap was used to calculate the different land use areas which are categorised into 41 classes including roads, pavements and a class that is predominantly gardens. Ten areas (0.5 km radius) in York and Sheffield were sampled to give average land use areas in these cities. A study undertaken in Merseyside had similar data. These data were used to give an average area of different land use types in a residential area.

Usage data was taken from a paper reporting on herbicide usage and from sales data from two companies supplying the home and garden herbicide market. It is estimated that 20-30% of households use herbicide A realistic worst case scenario for domestic use herbicide wash-off to surface waters was defined as a suburban development where many house frontages drain directly to the road network and then via storm drains or culverts to a local stream. In order to facilitate comparison of results from the different HardSPEC scenarios, the basic suburban catchment was kept the same as those of the Urban catchment: Catchment area = 10 ha; Tributary stream length = 316 m and Tributary stream width = 1 m. Data from previous sections were then used to define the percentage distribution of land cover types in the catchment, the length of roads and number of houses present, the area of domestic property likely to be sprayed with herbicide that could subsequently contribute to washoff to the tributary stream, and the runoff coefficients


from different land cover types.

It was necessary to estimate a weed density and therefore the area to which herbicide would be applied taking account of the jointing of the different hard surface types, plant interception was the same as other HardSPEC scenarios (10%), but drift was 0% as the runoff is draining via culverts and the stream is not directly exposed. These values were combined with usage data to give a total area treatment area.

The dynamics of herbicide fate in the tributary stream that is the target for regulatory risk assessment were exactly the same as those described for the urban and major road stream scenarios.

The HardSPEC model was physically adapted to incorporate a domestic use scenario. New cells were added in each worksheet (Herb_props, OUTPUT, Losses_BR, Masses lost per 0.5 mm rain, Losses_AR) to accommodate the data. In addition, a new worksheet “Domestic Use scenario” was added before the existing worksheet “Urban scenario”. This worksheet defines the surface characteristics of the domestic use catchment; the percentage of each hard surface type that is spot sprayed in a single domestic property; the herbicide application rate and percentage of each surface type in the catchment receiving spray in the peak application week, and the calculated area of each hard surface type receiving spray.

The new version of HardSPEC is version 1.4.1.

The model was run with six test compounds with a range of physico-chemical properties. The results were compared to those for the relevant surface water bodies in the HardSPEC urban and major road scenarios to see whether the potential aquatic exposure from domestic use of herbicides was similar to that from other hard surface uses by local, regional or national authorities. The comparisons indicated that peak daily aqueous phase PECsw for the six test compounds is between 6% and 88% smaller in the stream of the domestic use scenario than in that of the urban scenario. The differences appear to result mainly from the compound specific surface sorption coefficient with those compounds having the smallest concrete Kp value having the smallest differences and vice versa. Concrete surfaces comprise a major part of the domestic scenario but a much smaller component of the Urban and major road scenarios.

The domestic scenario parameters of the new version of HardSPEC (v1.4.1) are largely based on measured data, and incorporate a set of justified realistic worst-case characteristics. As such, the model provides a robust tool for calculating aquatic exposure for use in the regulatory risk assessment of herbicides used in the home and garden sector.

Note: Since completing project PS2237, further, minor amendments have been made to HardSPEC and these are reported under project PS2236.

Project Report to Defra8. As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with details of

the outputs of the research project for internal purposes; to meet the terms of the contract; and to allow Defra to publish details of the outputs to meet Environmental Information Regulation or Freedom of Information obligations. This short report to Defra does not preclude contractors from also seeking to publish a full, formal scientific report/paper in an appropriate scientific or other journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms. The report to Defra should include: the objectives as set out in the contract; the extent to which the objectives set out in the contract have been met; details of methods used and the results obtained, including statistical analysis (if appropriate); a discussion of the results and their reliability; the main implications of the findings; possible future work; and any action resulting from the research (e.g. IP, Knowledge Exchange).


1 IntroductionHardSPEC is a tier-one risk assessment model that can be used to predict the environmental concentration of herbicides in an urban and rural-road setting, i.e. hard surfaces. The model can be used to assist in decision making as to whether a new, product or the change-of-use of a current product, can be used in such environments without posing an unacceptable risk to aquatic life. At present the model scenarios address the use of herbicides by relevant professionals (e.g. local authorities and weed control contractors) in urban areas and along major transport routes. However, herbicides are also used on hard surfaces in the domestic home and garden sector and, as there is currently no HardSPEC scenario that addresses such use, the Hard Surfaces Steering Committee has identified this as a significant limitation of the model with respect to its regulatory use.

The main differences between use in the home and gardens and the use on urban/rural roads are the ratio of hard surface to non-hard surface, the availability of dilution water, and the hard surface type. These features can have a significant impact on the amount of herbicide transported to the drains, and the resultant herbicide concentration.

Previous work on the HardSPEC model has considered the development of a home and garden scenario (Project PS2221) but this was necessarily theoretical and the parameters were primarily based on assumptions. Consequently, this section of the model is not sufficiently robust, thus there can be little confidence in its output.

The aim of this study was to provide supporting data to underpin parameters of a home and garden scenario for HardSPEC and to develop such a scenario. (The study was only concerned with those products that may be applied to hard surfaces and it assumes that any pesticide falling on to a 'soft' surface (soil, grass) would not result in run-off to the street drains.

The objectives of the study were to:Define typical ratios of hard surface to non-hard surface in gardens.Establish the probable quantity of dilution water.Estimate the number of residences served by the same drain that will spray at one time.Develop a home & garden scenario for HardSPEC.Compare the output of the home & garden scenario to urban/rural road scenario in the current version of HardSPEC, and other data.

The methodology for each objective was discrete, thus each objective is detailed in a separate chapter. The study did not lend itself to an overarching discussion section, as the development of the scenario effectively collated the work from each chapter.

2 Garden layout A principle objective of the study was to define a ‘typical’ garden layout in relation to hard surfaces (patios, pathways and driveways) compared to ‘soft’ surfaces. A number of data sources were used to define a garden layout including:

BUGS (British urban garden survey), Ealing Council survey of front gardens A survey in Leeds A survey in Merseyside Survey of English Housing

Whilst other research has been conducted outside the UK, this was considered unsuitable for the current study as housing patterns are generally country-specific thus it would not be possible to use these data to produce a typical garden layout in the UK. A summary of the methodology for each study and the key relevant findings are given in the Appendix 'Garden Layout' and this Appendix is referred to as section 2.

The main findings from the data were that there is variation in garden size within and between cities but this could be expected given that the studies were undertaken in different years, in different cities and using different techniques.. However, the overall findings were broadly similar, giving confidence in their reliability, and they can be summarised as thus:

Garden sizes were decreased in the order detached > semi-detached > terrace The majority of houses have gardens Most back gardens have a patio Gardens represent 20 – 40% of the total land area in cities There has been a substantial increase in the paving over of front gardens in the last decade both in

terms of the number of properties and the area paved.


3 Dilution waterAny rainfall that generates runoff from roads, pavements, gardens and roofs, will be from both treated and untreated areas thus a proportion of the run-off will be dilution water. The amount of runoff will depend on the total area of hard surface and the ability of water to infiltrate the surface – defined by a runoff coefficient. It was therefore necessary to define a typical catchment, and within this, to quantify the proportion of different surface types. The area of land generating runoff, and the nature of the surface, was estimated using digitised versions of Ordnance Survey maps. The information held on the maps is detailed, and land areas are categorised into 41 classes, including ' General Surface Multi Surface Multiple Total ' (i.e. gardens,) roads and pavements as individual categories. The total area of each class within an area of land equal to a circle of 1 km diameter within the cities of York and Sheffield were determined; within each city, ten sample areas were randomly selected and the data collated, per city, to calculate an overall mean for each category.

The scientific literature was used to identify runoff coefficients associated with different hard surface types. A volume of runoff could then be calculated assuming 1 mm of rainfall on 1m2 gives 1L of runoff where the runoff coefficient is 100%.

Full details of the methodology and findings of the proportion of surface types and the runoff coefficients is provided in the Appendix 'dilution water' which is referred to as section 3.

4 Drainage and herbicide application characteristics 4.1 DrainageMany of the runoff coefficients for urban areas are based on empirical relationships between rainfall and the amount of water entering the sewage network. The runoff coefficient therefore already accounts for that water that seeps to groundwater and that which may reach the surface water drains.

Areas in front of the house will ordinarily be designed to take water away from the house to prevent damp. Impervious front gardens will commonly drain to the street where it is assumed that the water will end up in storm sewers. The drainage of back gardens is more complex. Modern building regulations require that water should drain to soakaway wherever possible and only enter surface water drains as a last resort. It is reasonable to assume that water draining from hard paths will be diverted to the edge of the path into the soil, otherwise the path would become a stream in wet weather and defeat the object of having the hard path. Drainage from patios may not necessarily drain to soil. Patios are invariably adjacent to the house and the design will be such to enable the water to dissipate quickly.

The high frequency with which patios were present (see Section 2) warranted further investigation into the nature of patios and their drainage. Patios typically comprise paving slabs in the UK although some may be bricked, but there are no data on the percentage of different surface types. The drainage of patios is recognised as important in order to prevent water ponding. Patio company websites (e.g. www.pavingexpert.com) indicated that, unless the underlying soil is highly impermeable, small patios can generally drain to a soakaway. However, as patios become larger, and thus the amount of runoff is larger, it may be necessary to install a drain that is linked to the surface water drain. Consequently, it is the larger patios that have the highest probability of being linked to surface water drains. There were no data on the number of patios linked to a surface water drain but it is considered that although this can occur it is not the normal situation.

4.1.1 Number of houses served by a single drainThe number of houses served by a drain was calculated from Yorkshire Water drainage maps which differentiate between combined and surface water sewers (Figure 1; blue lines = surface water drains).

The houses were counted along a length of drain that was subsequently measured. The scale of the map was then used to adjust the length accordingly. This was conducted for ten areas within York and seven areas in Sheffield. Areas were chosen to reflect different housing types.

The overall average number of houses per 100 m of drain length was 21. There were marginally fewer detached houses per 100 m of drain, but the difference between the housing types was only minor (Table 1).

Table 1 Mean number of houses served by 100 m of drain per housing type

York SheffieldDetached 19 17

Semi-detached 19 25Terrace 21 23


Figure 1 Example of a drainage map

4.2 Usage patternsTwo sources of data were used to characterise the usage patterns for herbicides in the domestic sector: a study of the use and storage of domestic pesticides in the Bristol Avon district and confidential EPOS (electronic point of sale) monthly sales data from two major companies that supply the Home & Garden market.

4.2.1 Use and storage of pesticides in the Bristol Avon districtTwo papers have been published reporting the results of this study (Grey et al, 2006; Nieuwenhuijsen et al, 2005) but only the data from Grey et al, 2006 has been used here as it is the most relevant for herbicide use. The paper reports the results of a screening questionnaire on pesticide usage and a more detailed series of targeted follow-up face-to-face interviews. The screening questionnaire was completed by 831 sets of parents involved in the Avon Longitudinal Study of Parents and Children (ALSPAC) project which aimed to examine the relationship between the environment and the development and health of children. The detailed follow-up study comprised face-to-face interviews with a randomly chosen sample of 147 parents, stratified by 1:2 self-reported pesticide users to non-users. This stratification was used to try and remove any bias from the screening questionnaire due to initial under reporting of pesticide use.

Results from the initial screening questionnaire indicated that 21% of the 831 parents reported using weed killers but in the follow-up interviews 27% of parents reported weed killer use, including some of those who had initially reported no use, confirming under-reporting of use in the questionnaire, mainly as a result of recall bias during the face-to-face interviews, or misunderstanding of the term pesticide. The frequency of use ranged from 1 to 18 applications per year, but the latter value is extremely unusual and the 25 th and 75th percentile frequencies were 1 and 2 respectively, indicating that most people who use domestic weed killers only apply once or twice a year.

One final piece of information from this study is of use here. Analysis of the detailed face-to-face interviews indicated that home owners were 3.5 times more likely to use 2 or more pesticides in the garden, as opposed to people who rent homes. This is important as it suggest that any realistic worst-case scenario will be in areas where most people are home owners.

4.2.2 Confidential EPOS monthly sales dataConfidential EPOS monthly sales data from two major companies supplying the Home and Garden sector in the UK was summarised to provide critical information on the timing and level of herbicide use within the year. Based on this data it is estimated that about 4 to 6 million units of herbicide are sold in the UK in a year. There are approximately 20 million gardens in the UK and thus market penetration for herbicide products is estimated to be between 20 and 30%. This correlates very well with the usage figure of 27% from the study reported in Grey et al


(2006) and gives confidence in the numbers produced from both sources.

The monthly market data also indicates that about 70% of purchases occur in the 120 days from the start of April to the end of July with peak sales in May. Assuming that application occurs soon after purchase, if applications were to be spread equally over the 120 day period, then 100 x 1/120 = 0.83% of the target gardens are sprayed on any one day. Clearly the market data indicates that applications are not spread equally over the 120 day period as there is a sales peak in May. However, this is offset by the fact that the 120 day period includes only 70% of product usage and also that not all households are likely to use products containing the same active ingredient. It is estimated that the most commonly used active ingredient is present in about 70% to 80% of herbicide products sold. Given these two factors, which would significantly reduce the real average daily usage figure in the 3 months of April to June, a realistic worst case for the percentage of target households (who purchase herbicides) applying a single active ingredient in any one day is 0.83%.

5 Home and garden scenario for HardSPEC The objective in developing a surface water scenario for assessing the aquatic exposure from herbicides used in the Home and Garden sector is to identify and characterise a realistic worst case situation where herbicides are likely to be applied to the hard surface areas of domestic properties within a short period of time and have significant potential to be washed off to a relevant aquatic water body over the subsequent few months. This was achieved by identifying a basic hydrological scenario based on a real suburban catchment, characterising the different types of land cover and surfaces present in it using the data presented in chapters 2 and 3 above and finally estimating a realistic worst case percentage of properties and surfaces that are likely to have herbicide applied within a week using the information presented in section 4.2.

5.1 The basic Hydrological ScenarioIn the Home and Garden sector the most common areas of herbicide usage are likely to be in suburban developments where properties are dominantly privately owned and have gardens (see section 4.2). As indicated in section 4.1, the main domestic property input to storm drains and thence to surface water bodies, is from property frontages, including gardens, which lead directly to roads. A realistic worst case scenario for domestic use herbicide wash-off to surface waters would thus be a suburban development where many house frontages drain directly to the road network and then via storm drains or culverts to a local stream. Such a situation is illustrated in Figure 2. It is based on a real location where a small headwater catchment has been built over with residential developments; the existing headwater stream has been enclosed in a culvert into which most of the road drains empty and which outfalls directly into a stream tributary of a small river. The tributary stream contains natural vegetation and is equivalent to the ‘edge-of-field’ water body that forms the target for regulatory risk assessment.

5.2 Characteristics of the Domestic Use catchmentHaving established the basic worst-case hydrological catchment, its detailed surface characteristics were defined as given in the following sections.

5.2.1 Catchment area and associated surface water body.In order to facilitate comparison of results from the different HardSPEC scenarios it is important to keep them as consistent as possible. The basic suburban catchment described above is most similar to the urban catchment - its size and associated water bodies are therefore kept the same as those of the Urban catchment:

Catchment area10 haTributary stream length: 316 mTributary stream width 1 m

5.2.2 Percentage distributions of land cover types within the catchment.The percentage distribution of different land cover types within the catchment were defined from the calculated average value for each type in the York and Sheffield city areas (Section 3.1), and Merseyside (Section 2.4). This data is shown in Table 2.

Table 2 Derivation of percentage cover of land types in the domestic use catchmentLand cover type York City Sheffield City Merseyside Average

Buildings 19.2 17.5 27.2 21.3Roads and tracks 10.8 11.2 11.0 11.0Roadside hard surfaces 4.0 5.2 4.6 4.6Roadside natural surfaces 2.4 2.0 2.2 2.2Other hard standing surfaces 7.3 5.4 6.4 6.4Other natural surfaces 17.4 12.9 15.1 15.1Gardens 38.9 46.0 33.4 39.4


The catchment is denoted by the black, dashed line

Figure 2 Overview of the domestic use catchment

The three areas clearly have a very similar percentage cover of roads, their adjacent surface types and other man made surfaces (interpreted here as areas of hard standing). This is important because, for the domestic use scenario, roads and other non-domestic hard surface areas provide the largest amount of runoff not directly associated with herbicide application and thus available for dilution of herbicide loads washed off domestic hard surface areas. The uniformity of their percentage cover in the three areas means that the average data can be used with confidence to characterise the domestic use scenario.

The main differences between the three areas occur in the percentage cover for buildings, ‘natural’ surfaces and, in particular, gardens. Sheffield has by far the largest percentage cover of gardens and Merseyside by far the largest cover of buildings although there is no data to quantify what proportion of these are domestic buildings. The worst-case nature of the catchment with respect to domestic use of herbicides is based on the presence of a large number of properties with hard surface areas that runoff or drain into surface waters. This depends on the length of roads that carry storm drainage and/or feed into the culverted former headwater stream, the number of properties associated with such roads and the area of relevant hard surfaces associated with those properties. None of these factors are dependent on the percentage cover of buildings, natural surfaces or gardens and thus using an average value for these surface types does not affect the worst case nature of the scenario.

The percentage cover of basic land types in the catchment is therefore defined from the average values of the York city, Sheffield city and Merseyside data shown in Table 2. above.


5.2.3 Length of roads and number of houses present.The length of roads and number of associated housing properties in the catchment is an important characteristic as it determines the number of front gardens in the catchment that may contain hard surface areas that run off directly to the surface water network. This information was derived from the size of the catchment, the average percentage cover of land defined as ‘roads’ in

(11%) and an average road width of 7.3m, which is the recommended width of a two-way single lane all-purpose urban road according to the Highways Agency Design Manual for Roads and Bridges (Highways Agency, 2005, figure 4-4a, page 4-17).

Length of road network in the catchment = 100000 m 2 x 11/100 = 1506.8 m 7.3 m

The number of house properties associated with this length of road network is then defined from the data given in section 4.1.1 that there are, on average, 21 houses associated with each 100 m of drained road. As discussed in section 4.1.1, most but not all, of houses are associated with roads that have surface water storm drains. It is therefore acceptable to calculate the number of houses along the road network with this value but then reduce that number to take into account the small lengths of road that are not connected to the storm drainage network:

Total number of houses along the road network = 1506.8 x 21 = 316 100

5.2.4 Areas of domestic property hard surface types likely to be sprayed with herbicide and have subsequent runoff to the tributary stream.

Having established the length of road network in the catchment and the number of houses associated with it, the critical factors in establishing the realistic worst case scenario for domestic use of herbicides are the areas of different hard surface types likely to receive herbicide spray and subsequently contribute run off to the road network and its associated surface drainage.

Firstly, the number of houses that have gardens was identified using the information from the Survey of English Housing data (see Table 9, section 2.5). This indicated that at least 99% of detached and semi-detached houses have gardens and that at least 95% of terraced houses built after 1946 also had gardens. Lower percentages of properties with gardens only occurred in terraced houses built before 1946 and flats. The BUGS I data did indicate that only 87% of houses in their survey had gardens (Section 2.1) but this study focussed on rear gardens only and therefore is not used here. As indicated in section 5.1, to comply with a realistic worse case for domestic usage, the scenario catchment is dominated by suburban housing and, as such a scenario is very unlikely to contain the type of terraced housing built before 1946, the number of houses with gardens is set at 99%, being dominated by detached and semi-detached properties. It is assumed that houses with no gardens are not subject to domestic use of herbicides even if they are situated in road areas that are linked to the surface drainage network.

Next, to take into account the small amount of suburban road that is not associated with a surface water storm drainage system (see section 5.2.3) it was estimated that 95% of the total 1506.8 m of road length in the catchment was linked to storm drains, giving a length of 1431.5 m. Using the figure of 21 houses per 100m of road (see section 1.2.3 above), this gives a total of 301 properties associated with the road drainage network, of which 298 (99%) have gardens. To comply with the realistic worst case conditions, all of these properties have some sort of front gardens that may have hard surface areas draining directly to the road drainage system. Other properties that do not have gardens or are along road sections that do not have storm drains do not contribute runoff loads from domestic herbicide use and need not be considered further other than their contribution to hydrological runoff from the different land cover areas in the catchment.

The hard surface area covering the front gardens in each of the 298 properties was based on data from the Ealing and Leeds studies (sections 2.2 & 2.3 respectively). In the Leeds study the average paved front garden size was 53.4 m2, whereas in the Ealing study, the average size of front gardens was 41 m2 and the average percentage of this that carried hard surfaces was 64%, giving a hard surface area of 26.24 m2. The average of these two values gives a front garden hard surface area of 39.8 m2. It is recognised that both the Leeds survey data and the Ealing data may under-estimate the actual hard surface area of front gardens because, in the Leeds study, areas where there were any doubts concerning the nature of the surface type were considered to be porous, whereas in the Ealing study, drives were specifically excluded from the data. Nevertheless, the difference between the two hard surface areas (53.4 and 26.24 m2) suggests that the Leeds data easily compensates for the exclusion of drives from the Ealing data. The combined average is thus likely to represent a reasonable overall average hard surface area for front gardens in the proposed scenario. Use of such average data, rather than trying to estimate a worst-case hard surface coverage, is consistent with the principles for characterising sub-scenarios described in the introduction to this chapter because the basic suburban scenario identified is already a realistic worst case scenario for domestic use herbicide wash-off to surface waters (95% houses along the road network Have frontages that drain directly to the road storm drains or culverts and thence


to a local stream).

Finally, detailed data from the Ealing study (Section 2.2) was used to establish the average percentage cover of the four principal types of hard surfaces used in front gardens. This data is shown in Table 3 and it is used along with the average area of front garden hard surface and the number of houses with front gardens adjacent to drained roads to calculate the areas of each relevant front garden hard surface type in the catchment.

Table 3 Percentage cover of different types of front garden hard surfaces (from the Ealing study) and the calculated total areas of these contributing runoff to the catchment drainage network.

Surface type Average % cover in a front garden

Total area in the catchment m2

Bricks 33 3823.6Concrete & paving slabs 56 6488.4Asphalt 7 811.1Gravel 4 463.5Total 100 11586.5

One more factor is needed to ensure a realistic worst case for domestic use of herbicides. As stated in section 4.1, some patio areas in rear gardens may be linked to the storm drainage network and thus potentially contribute to domestic use herbicide runoff loads to surface water. The BUGS data (section 2.1, Table 2) contains statistics on the areas of patios in rear gardens and indicates that 93% of rear gardens have patios which have an average size of 38.6 m2. Based on the calculated 298 properties that have gardens and are adjacent to roads with storm drains, this gives a total rear garden patio area of:

298 x 0.93 x 38.6 = 10692 m2

However, it is unlikely that a large number of patios will be linked to the storm drainage system. No data exists to establish a robust quantification of this but a value of 10 % is used in the scenario and this is considered to be a conservative estimate. The total area of patio that drains to the catchment surface water network is thus:

10692 x 0.1 = 1069.2 m2

Most patios consist of some sort of concrete based paving, either as whole ‘slabs’ or ‘crazy paving’. The calculated relevant area of patio thus needs to be added to the calculated area of front garden concrete and paving slabs to derive the total area of concrete surfaces contributing runoff to the surface water network in the catchment

Total area of concrete surfaces contributing runoff = 7703.5 m2

5.2.5 Rainfall percentage runoff coefficients of the different land cover types in the catchment and associated herbicide wash-off losses.

In order to calculate the total volume of runoff moving to the tributary stream that forms the target for regulatory risk assessment (see section 5.1), each surface type in the catchment has to be assigned a rainfall percentage runoff coefficient. These values are based on the data given in section 3.2. The values used and their derivation are summarised in Table 4.

The values in Table 4 represent the overall rainfall:runoff ratios from different surface types and indicate that not all the rain falling on such surfaces runs off. Some of the rain falling on surfaces is lost through evaporation or wind-blown losses whilst other losses could be via ‘leakage’ of runoff through surface cracks and joints or retention within surface depressions. Such losses are important with respect to the herbicide loads washed off each surface during runoff as any ‘leakage’ from the surface would result in a reduction in the herbicide loads washed off to the drainage network. For each relevant hard surface type, it is thus necessary to estimate the amount of leakage that is likely to occur so that it can be subtracted from the calculated wash-off loads.

Table 4 Percentage runoff coefficients for different land cover types in the domestic use catchment.Land cover type % runoff Derivation

Building roofs 68 Average measured runoff from building roofs in the months of March, April & May from Ragab et al 2003b.

Concrete paving 65 Hollis et al, 2008.Brick paving 50 Average value from the brick paving studies.Asphalt 75 Highest value from the study by Ramier et al, 2003.

Gravel 20Value set to the same as that for soft surfaces – Hollis et al, 2008 indicated that runoff from gravel was not generated under the conditions of the study.

Soft surfaces 20Value used for the agricultural field in the Major Road scenario which uses the smallest SPR coefficients for all soil types that occur adjacent to surface watercourses.

All data/references from section 3.2.


There are three relevant hard surface types: asphalt, concrete and bricks. The rainfall:runoff coefficients used for each of these are based on controlled wash-off measurements from small areas of each surface type. Of these, asphalt forms the most coherent surface which contains no cracks or joints and thus any rainfall not lost as runoff must be lost via evapotranspiration or retention in surface depressions. The rainfall:runoff value for asphalt (75%) thus represents a base-line for non-runoff losses from the system that does not include leakage through cracks or joints in the surface type. As a result, the difference between the rainfall:runoff value for asphalt and those for concrete and bricks represents additional losses from each surface type that occur through ‘leakage’ and do not contribute to herbicide loss to the drainage network. These values are shown in Table 5 and are taken into account in the model when calculating the total mass of herbicide washed off to the tributary stream of the domestic use catchment.

Table 5 Percentage rainfall:runoff coefficients for hard surface types in the domestic use catchment that receive herbicide spray and their associated ‘leakage’ losses via cracks and joints.

Hard surface type receiving herbicide spray

Percentage rainfall runoff Percentage of ‘leakage’ from the surface via cracks / joints

Asphalt 75 0Concrete paving slabs 65 10

Bricks 50 25

5.2.6 Summary of the calculated areas of different land cover in the catchment.Based on the data and calculations described in sections 5.2.1 to 5.2.5 above, the defining land cover characteristics of the domestic use scenario are detailed in .Table 6 Percentage runoff coefficients for different land cover types in the domestic use catchment.

Characteristics Value % cover Derivation

Area of asphalt road 10999.6 m2 11.00Average % cover of roads from

Area of asphalt footpaths 3925.5 m2 3.93Average % cover of roadside hard surfaces from , minus the area of concrete kerb stones.

Area of asphalt drives (receiving herbicide) 829.3 m2 0.83 Front garden hard surface data from

Table 3.Total asphalt area draining to stream 15754.4 m2 15.75 Sum of the three asphalt surface areas.

Area of concrete kerb stones 662.99 m2 0.66Calculated length of roads from section 5.2.3 and the size of kerb stones used in the urban & major road scenarios.

Area of other concrete surface not receiving herbicide 6400.0 m2 6.40

Average % cover of other hard standing surfaces from .

Area of concrete drives & patios connected to drains (receiving herbicide)

7703.5 m2 7.70Front garden hard surface data from Table 3 and the total area of patio that contributes runoff to the road drainage.

Total concrete area draining to stream 14766.5 m2 14.77 Sum of the three concrete surface areas.

Area of brick drives draining to stream (receiving herbicide) 3909.5 m2 3.91 Front garden hard surface data from

Table 3.

Area of Roofs 21304.8 m2 21.30 Average % cover of buildings from .

Area of soft surfaces & gravel 44264.9 m2 44.26

Sum of the average garden, roadside natural surface and other natural surface areas from minus the hard surface areas receiving herbicide

Total catchment area 100000 m2 100.00 Same as that of the Urban scenario

Stream length 316 m - Same as that of the Urban scenario

5.3 Identification of herbicide Application characteristics

In the domestic situation, herbicide may be applied to all garden surfaces, soft and hard, but for the purposes of scenario development, only those hard surfaces which contribute runoff direct to the catchment surface water network need to be considered. It is assumed that all application or wash-off to all other surfaces does not contribute any significant load to the surface water stream that is the regulatory target for environmental risk assessment. The critical factors determining pesticide loads available for wash-off are therefore the amount of herbicide impacting on individual surfaces contributing wash-off and the percentage of these surface types that


are likely to be sprayed on or about the application day.

5.3.1 Percentage of properties to which herbicide is appliedThe data presented in section 4.2 indicated that, on average, about 30% of UK households with gardens are likely to use herbicides. However, the realistic worst case scenario that has been developed comprises a suburban catchment dominated by owner-occupied properties and the study reported by Grey et al (2006) indicated that such households are much more likely to use pesticides in the garden than are other households. It is therefore estimated that, in the suburban catchment simulated by the model, 50% of those households with hard surfaces that contribute runoff direct to the surface water network will use herbicides on them in any one year.

The HardSPEC model can only simulate the fate of herbicide applied on a single day but it is unrealistic to assume that herbicide applications applied on subsequent days do not contribute loads to the surface water network. In order to simplify the input data and retain a first tier approach to the exposure estimation, application loads contributing to surface wash-off are maximized by assuming that herbicides are applied to relevant properties in the catchment over a succession of rain-free days within the peak application month. During this rain free period any applied herbicides are retained on the surface. Although, as suggested in a monitoring study of herbicides applied to a major road (Heather et al, 1998), some degradation may occur, the great majority of the accumulated load applied during the rain free period will be available for wash-off.

To identify a realistic worst case for the number of rain-free days likely to occur during the application period the six daily weather data sets used to derive the realistic worst-case rainfall pattern for the HardSPEC model were analysed. The results are shown in Table 7 and indicate that within the spring application period, whereas a period of 7 rain-free days is likely to occur almost every year and a period of 14 rain-free days about one year in 3, a period of 21 rain-free days is only likely to occur 3 times in 100 years. A realistic worst-case for the rain-free period for the scenario is thus between 14 and 21 days and a period of 18 days has been selected as it represents a 1 in 10 year frequency (90th percentile worst-case).

Table 7 Statistical analysis of the frequency of rain-free periods at 6 weather stations representative of England & Wales.

StationNumber of

years in period

Number of years with a rain free period of:

> 7days % > 14 days % > 18

days % > 21 days %

Brighton 23 21 91.3 7 30.4 2 8.7 0 0.0Cambridge 23 20 87.0 10 43.5 4 17.4 2 8.7

Keele 23 18 78.3 7 30.4 1 4.3 0 0.0Lowestoft 23 20 87.0 7 30.4 1 4.3 1 4.3

Newton Rigg 23 19 82.6 6 26.1 2 8.7 0 0.0Swansea 22 19 86.4 8 36.4 4 18.2 1 4.5

Average 19.5 85.4 7.5 32.9 2.3 10.3 0.7 2.9

In order to ensure maximum wash-off of herbicide in the scenario, the rainfall pattern that initiates wash-off following the final application of herbicide has events occurring on each day for 12 days and it is reasonable to assume that no herbicide is applied to domestic properties during this ‘rainy’ period. Any herbicides that are applied following the rainy period will of course add to the loads washed off from residues of the original applications. However, the amount of ‘new’ herbicide applied will be significantly smaller than the amount related to the peak application month and the realistic worst-case rain-free period. Any residual wash-off loads will also be very small compared to those of the initial wash-off events. As a result any peaks in surface water concentrations resulting from later herbicide application will be much smaller than those relating to the main application period and the realistic worst-case nature of the proposed scenario is thus maintained.

The information in section 4.2.2 indicated that the peak month for herbicide purchase is May, during which approximately 25% of the total annual purchases of the most popular product were made. If the identified 18 day rain-free period occurs in the month of maximum purchases, the majority of individual products purchased is likely to be applied during this spell. A value of 20% has been selected to represent the percentage of households estimated to use pesticides that are likely to apply herbicides during the 18 rain-free days in the peak usage month. The 5% difference from the monthly peak purchase value is to take into account the small number of households that purchase herbicides in the month but do not apply them and also the small amount of degradation that will occur in compounds applied at the start of the rain-free period.

Combining the estimated percentage of households likely to use herbicides in any one year with the estimated peak percentage of those households that will apply herbicide, the percentage of properties that contribute hard surface runoff directly to the surface water network in the catchment and have herbicides applied within an 18 day rain-free period is:

50 x 20/100 = 10%


5.3.2 Amount of herbicide sprayed on individual surfacesIn the domestic use situation, herbicide is normally spot-applied from a hand-held sprayer. There may be some locations, such as long joints between surfaces, where there is an effectively continuous swath applied but this will still be from a hand-held sprayer. What determines the amount of spray applied is thus the coverage of weeds likely to be present. As with other HardSPEC scenarios, a moderately severe weed infestation is assumed and the coverage of weeds will be largely determined by the amount of joints or cracks present over the hard surface area.

Concrete and paving slabs: Paving slabs are approximately 60 cm x 60 cm and thus have an unbroken surface area of 0.36 m2. Not all concrete surface types will be made of paving slabs as some will be ‘crazy paving’ with smaller unbroken surface areas and others complete spreads of concrete with only a few cracks and crevices. On average however, these two different types are likely to balance out and it is thus reasonable to base an estimate of the area of joints/cracks carrying weeds on paving slab dimensions. The average hard surface area of a front garden is 39.8 m2 (see section 5.2.4 above) and, assuming the coherent concrete surfaces in this area are approximately square, this gives a total number of 97 joints/cracks, each 0.6 m long, with the potential to carry weeds and thus receive spray. However, even with a moderately severe weed infestation not all of these joints will carry weeds. A more realistic worse case is that 50% of the joints/cracks will have some sort weeds in them. It is assumed that along these joints/cracks there are, on average, 4 weeds per 0.6 m and that the impact area of a single spot spray has a diameter of 0.15 m (radius 0.075 m). Within a single concreted garden area therefore the total area receiving spray is:

97 x 0.5 x 0.6 m x {4 x (PI x 0.0752)} = 3.428 m2

The percentage of concrete surface receiving spray is thus:

100 x 3.428/39.8 = 8.7%

Allowing for errors in estimation this figure has been rounded up to 10%Asphalt: Within asphalt areas, weeds colonise surface cracks and depressions. The number of such features is likely to vary widely and it is not sensible to attempt the sort of calculations carried out for concrete surfaces. As a reasonable alternative therefore, the percentage of asphalt hard surface per property receiving spray is set to 10% the same as that calculated for concrete.

Bricks: Brick or block paving areas have very many joints because of the small size of the individual blocks. Assuming a spot spray impact area of 0.15m diameter, the whole hard surface area has the potential to receive spray and thus the actual percentage sprayed is dependent solely on the level of weed infestation. The domestic use scenario has a basic assumption of treatment for moderately severe infestations and thus, as with the concrete surface, 50% of the joints are assumed to carry weeds and, in this surface type, this means that 50% of the hard surface receives spray per property sprayed.

Gravel: Gravel surfaces have the potential for significant weed infestation and thus to receive herbicide spray but, as they do not contribute runoff directly to the surface water network (see Table 4 above), need not be considered further.

Based on the above estimations, the area of each hard surface type that receives spray and contributes runoff directly to the surface network is calculated from the area (m2) of the surface type, the percentage of the surface type receiving spray per property sprayed and the peak percentage of properties likely to be sprayed in any 7 day period. These values are given in Table 8.

Table 8 Data used to calculate treated area contributing to herbicide washoff

Surface type Total area in catchment m2

Percentage of surface receiving spray per property sprayed

Peak percentage of properties sprayed in a 7

day period

Area receiving spray m2

Concrete 7703.5 10 3 22.6Asphalt 829.3 10 3 2.4Brick 3909.5 50 3 57.3

5.3.3 Plant interception and spray driftOf the herbicide sprayed onto each surface type, 10% is intercepted by growing plants and not subsequently washed off, exactly as described in the existing HardSPEC urban and major road scenarios. With any scenario involving spray application, some drift of the applied spray occurs. However, because the defined scenario is one of domestic use in gardens of suburban properties that are located some distance from the target surface water body (see sections 5.1 and 5.2 above), there can be no spray drift to this surface water body. It is therefore assumed that any spray drift that occurs still impacts on some part of the surface type to which it is applied, thus maximising the amount of herbicide impacting on that surface.

The plant interception percentage is used together with the compound application rate and the area of each surface receiving spray to calculate the mass of herbicide impacting on each surface type and available for


wash-off direct to the surface water network.

5.4 Herbicide fate in the surface water body

In keeping with the principles used to develop the existing HardSPEC model, the dynamics of herbicide fate in the tributary stream that is the target for regulatory risk assessment are exactly the same as those described for the urban and major road stream scenarios (Hollis et al, 2004):

1. On the day of application, there is no runoff and the pond contains its specified minimum volume of water. There is also no spray drift to the water body which thus contains no herbicide in either the water or sediment phases.

2. On the first day after application, rainfall occurs and catchment runoff contributes both water and associated herbicide loads as inputs to the stream. As with the ‘STEPS1-2 in FOCUS’ model, herbicide input loads are separated into sorbed and non-sorbed phases, according to the soil Koc, the calculated daily water depth in the pond (taking into account the runoff volumes), effective sediment depth, sediment bulk density and sediment organic carbon %.

3. At this stage there is no residual mass in the surface water body and so the final mass in the stream water and sediment phases at the end of this day is the same as the input mass

4. At the start of each subsequent daily time-step, no residual compound remains in the stream water phase as it has all removed in the previous day’s time-step.

5. The amount of residual compound in the stream sediment phase is calculated from the compound degradation rate applied to the final sediment mass of the previous day minus 2/3 of the sediment-phase input load from the previous day. This is because the rate of flow in the stream is assumed to be fast enough to remove 2/3 of the runoff input load coming in as sediment phase.

6. Only 1/3 of the daily runoff input loads are subject to partitioning during each time-step. As with point 5 above, because the stream water is dynamic, it is assumed that 2/3 of the mass input as runoff is either too far away from the stream sediment or is moving too rapidly to be subject to partitioning. This applies to inputs in both the water and sediment phases.

7. Daily concentrations in both water and sediment phases are calculated from the final masses of compound following partitioning. This is done to ensure that the concentrations reflect the maximum masses of compound present in the stream before 2/3 of the input sediment mass and all of the aqueous mass are removed by its daily turnover dynamics. If concentrations were to be calculated from the initial values at the start of the time step, as is done for the pond, they would be lower.

5.5 Adaptation of HardSPEC to incorporate a Home and Garden use scenario

In order to incorporate the methods developed and described in section 5.2 the following modifications to the existing HardSPEC model (v. 1.3.4) were implemented.

5.5.1 Worksheet “Herb_props” A new row was added to the Herbicide properties list below existing row 5. The title for the new cell B6 was added as ‘% of domestic scenario areas treated’. The adjacent cell (C6) was then given the value of ‘3’. This value is based on published survey data and on confidential company monthly sales data as explained in section 5.3.1 above. Although it can be altered by users, this should not be done unless there is very good sales evidence for a different level of peak usage in the spring application period simulated by the model.The format of the modified input data worksheet “Herb_props” is shown in Figure 3 below. Note for Users: For domestic use products, particularly those that are ‘ready to use’ formulations, details of compound contents in mass per litre may not be on the label nor may there be clear recommendations as to the dose to be applied per unit area. Model users thus need to pay particular attention to how they derive the application amount used as input to the model and give an argued justification for the amount used.

5.5.2 Worksheet “OUTPUT” A new line has been added to the ‘Acute Concentrations’ table (cells A10 to E10) to show the peak daily concentration in water and sediment in the domestic scenario stream. In addition, the daily PEC results for the stream water and sediment phases have been added to the graphics showing ‘water concentrations’ and ‘sediment concentrations’ for the “domestic stream” scenario. These are derived from cells BG12 to BG119 and BJ12 to BJ119 in the worksheet “Losses_AR”. Finally, a new cell, F10, has been added to the “Application day PECsw from spray drift” table (Cells F7 to F11) to indicate that there is no spray drift impact on surface water in the domestic use scenario.


Herbicide propertiesHerbicide name atrazine% of applied amount impacting as spray drift. Urban & Major road 2.8% of applied amount impacting as spray drift. Railway 0.091% of domestic scenario areas treated 3Measured Kp asphalt (mg m-2 ) 2.53Measured Kp concrete (mg m-2 ) 1.280soil koc (mL g-1 ) 100solubility (mg L-1 ) 33Specific Gravity 1.23DT50 in soil (days) 50DT50 on hard surfaces (days) not knownDT50 in sediment (days) 50DT50 in water (days) 2Application amount (g/ha)urban 3000Road 3000Railway 3000Fraction of 400 m2 railway track target area actually sprayed 1Run-off attenuation factor applied to leached load from ballast 1

Users must only change the fraction of target area sprayedFraction of 100m2 target area spot sprayed 1Input mass(g) from hand-held spot spraying 0.8400Concentration (mg l-1) in water phase 28.0000

These cells can be used to examine the surface water exposure in the Railway ditch resulting from application by a hand-held sprayer.

Figure 3 Modified input data worksheet “Herb_props” for HardSPEC 1.4.2

5.5.3 New Worksheet “Domestic Use scenario” A new worksheet “Domestic Use scenario” has been added before the existing worksheet “Urban scenario”.

This worksheet defines:

The surface characteristics of the domestic use catchment (cells B5 to D23);The percentage of each hard surface type that is spot sprayed in a single domestic property (cells H5 to I10);The herbicide application rate (derived from cell C17 in the “Herb_props” worksheet) and percentage of each surface type in the catchment receiving spray in the peak application week (cells E5 to F10);The calculated area of each hard surface type receiving spray (cells E19 to F22).

Derivation of the data in the worksheet are given in sections 5.1 to 5.4 above and the values are used to define fixed scenario input data to drive the various herbicide fate components of the HardSPEC models in the worksheets “Losses_BR”, “Masses lost per 0.5 mm rain” and “Losses_AR”.

5.5.4 Worksheet “Losses_BR” An extra column ‘F’ has been added to the sheet to included calculated values for:

“Amounts of herbicide sprayed (g)” on different surface types including brick blocks (cells F8 to F14).“Amount intercepted by plants (g)” on different surface types including brick blocks (cells F15 to F21).“Amounts lost in drift (g)” on different surface types including brick blocks (cells F22 to F28), although for the domestic use scenario these values are always 0.“Actual amount of herbicide falling onto the surface (g)” for different surface types including brick blocks (cells F29 to F36).


These values are used to calculate the amount of herbicide present on each 0.54 sq. m. area of different hard surface types in cells E178 to G178 of the worksheet “Masses lost per 0.5 mm rain”.

5.5.5 Worksheet “Masses lost per 0.5 mm rain”Calculations of surface wash-off losses for hard surface types in the Domestic use scenario have been added to this worksheet as follows:

Time series calculations of wash-off losses from asphalt surfaces have been inserted as rows 48 to 68 and from concrete surfaces as rows 118 to 138.

In order to drive these calculations, the ‘Mg reaching each 0.54 sq. m. of surface’ for different surface types have been added to cells E176 to G178.

Finally Catchment loss calculations for the Domestic scenario have been added as rows 192 to 194. For this purpose wash-off mechanism on brick surfaces in the domestic catchment are assumed to be identical to those from concrete surfaces and the calculations in rows 118 to 138 have been used for both surface types, although catchment level losses for brick include a reduction of 25% to take into account the ‘leakage’ losses via joints in the surface.

5.5.6 Worksheet “Losses_AR” Additional calculations have been added to this spreadsheet in order to calculate the daily water and sediment phase concentrations in the domestic use scenario stream.

Firstly, cells I2 to J9 have been used to define the percentage runoff characteristics of each surface type present in the domestic catchment. These values are explained in section 5.2.5 above. Cells Y6 to Z10 have also been used to define the number of 0.54 sq. m. blocks of concrete, brick and asphalt to which domestic use herbicides are applied. These data are used in row 193 of the worksheet “Masses lost per 0.5 mm rain” to calculate catchment level herbicide losses for the domestic scenario.

Secondly, calculations of the daily runoff volumes from each surface type in the domestic catchment have been inserted as cells K14 to N89 (hard surfaces) and Q14 to Q89 (soft surfaces). Based on these calculations of the volume of water through the domestic stream per day and the depth of water in the domestic stream have been inserted as cells AA14 to AA89 and AD14 to AD89 respectively.

Next, calculations for the domestic scenario, of the total mass of herbicide lost per rainfall event, the accumulated loss as a percentage of applied and the total mass entering the water body, have been inserted as cells AI14 to AI89, AL14 to AL89 and AO14 to AO89 respectively.

Finally those cells covering daily input masses in the stream aqueous and sediment phases, residual masses in the stream sediment phase, final masses in the stream aqueous and sediment phases and daily concentrations in the stream aqueous and sediment phases have been expanded to include calculations for the domestic use catchment stream. These are now in cells AR14 to AR89, AU14 to AU89, AX14 to AX89, BA14 to BA89, BD14 to BD89, BG14 to BG89 and BJ14 to BJ89.

5.5.7 HardSPEC Version Control The new version of HardSPEC incorporating the modifications and additional worksheets to simulate surface water exposure resulting from herbicide application in the Home and Garden use sector is numbered as follows:

HardSPEC Version number: 1.4.1.It incorporates scenarios covering herbicide use in urban areas, major road networks, railway lines and domestic situations. Model output includes predicted environmental concentrations in streams or ditches that receive runoff from urban areas, suburban areas, major roads or railway lines, as well as in receiving ponds designed as Sustainable Urban Drainage Systems. It also includes predicted environmental concentrations at a groundwater abstraction well head that is adjacent to a railway track.

5.6 Summary of ‘worst case’ assumptions in the Home and Garden scenario

The following ‘worst-case’ assumptions, some of which were developed for other HardSPEC scenarios, are incorporated into the newly developed scenario:

5.6.1 The scenario catchment The catchment represents a 10 ha suburban development where almost all the properties are owner

occupied detached or semi-detached house (identified by surveys as a worst-case for likely herbicide usage).

Many house frontages drain directly to the road network and then via storm drains or culverts to a local stream (a realistic worst case for wash-off to semi-natural surface water bodies).

5.6.2 Rainfall patterns


Significant rainfall (5mm) occurs on the day after application.

A 75th percentile wettest spring rainfall sequence follows application.

5.6.3 Herbicide application 50% of households that have runoff to the surface water network use herbicides in any one year. This is

20% higher than the average percentage of households that use herbicides identified from surveys and sales data.

Herbicides are applied during an 18 day rain-free period and all of this applied amount reaching hard surfaces is available for wash-off. The rain free period represents a 1 in 10 year event and thus a 90 th

percentile worst-case application period.

20% of those households using herbicides apply them within the 18 day rain-free period. This percentage is estimated using data for the peak month for sales.

Herbicides are used to control a moderately severe weed infestation and thus applied to targets along most joints or cracks in individual hard surface types.

5.6.4 Run-off and wash-off Percentage rainfall:runoff coefficients for different surface types in the catchment are based on

measured data related to the application months of March, April and May and include evapotranspiration losses relevant to those months, thus giving the lowest realistic runoff volumes for dilution of washed off herbicide loads.

All herbicide loads washed off individual surfaces move to the catchment stream, except for a small percentage that is lost from ‘leakage’ through the cracks or joints in concrete or brick paving surfaces.

5.6.5 Fate in the surface water body The surface water body is a small 1 m wide stream with characteristics similar to those of the FOCUS sw

streams but with a length of 316 m, consistent with a 10 ha catchment area. This minimizes the initial volume of water in the stream that is available for dilution of incoming herbicide loads.

6 Home and garden scenario comparisonThe newly developed home and garden use scenario of the HardSPEC model described in section 5 was run for the six ‘test compounds’ used in the laboratory and field monitoring studies used to develop HardSPEC. Basic physico-chemical characteristics and application rates for these compounds are shown in Table 9.

Table 9 Basic properties of the six Hard Surface ‘test compounds’.

atrazine diuron oryzalin oxadiazon isoxaben glyphosatesoil koc (mL g-1 ) 100 218 625 3200 767 28000solubility (mg L-1 ) 33 36.4 2.4 10 1.5 116000DT50 in soil (days) 50 100 63 60 105 47DT50 in sediment (days) 50 100 63 60 105 47DT50 in water (days) 2 2 2 2 2 10Application amount (g ha-1) 3000 2700 1730 4500 75 1800

Model results were then compared to those for the relevant surface water bodies in the HardSPEC urban and major road scenarios to see whether the potential aquatic exposure from domestic use of herbicides was similar to that from other hard surface uses by local, regional or national authorities. Peak 24 hour aqueous phase PECsw results are shown in Table 10 whereas the daily time series PECs in the aqueous phase of the water bodies are shown in Figure 5 to Figure 10.

Table 10 Comparison of peak aqueous phase PECsw for HardSPEC surface water scenarios

CompoundUrban Major road Domestic use

stream pond stream stream

Peak 24 hour PECsw aqueous phase (mg L-1)

atrazine 115.97 10.58 129.73 84.58diuron 97.05 8.45 96.80 60.02oryzalin 22.75 1.20 24.71 21.45oxadiazon 20.01 1.74 29.32 13.10isoxaben 2.74 0.23 2.49 2.10glyphosate 8.01 0.90 11.73 0.99

The comparisons indicate that, peak daily aqueous phase PECsw for the six test compounds is between 6% and 88% smaller in the stream of the domestic use scenario than in the urban scenario. The differences appear to result mainly from the compound specific surface sorption coefficient with those compounds having the smallest concrete Kp value having the smallest differences and vice versa (see Figure 7). Concrete surfaces comprise a


major part of the domestic scenario but a much smaller component of the Urban and major road scenarios.

Relationship between Kp concrete and ratio between peak PECsw domestic and urban scenarios

y = 1.0891e-0.2118x

R2 = 0.9816

y = -0.0761x + 0.8876R2 = 0.9108

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 2 4 6 8 10 12

Kp concrete

Rat

io

Figure 4 Relationship between surface specific Kp for concrete and the ratio between peak PECsw in the domestic and urban scenario streams

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80

Days after application

PEC

sw a

queo

us p

hase

mg

l-1

Domestic ditchUrban streamMajor road stream

Figure 5 Comparison of aqueous phase PECsw for atrazine in the domestic use stream and the urban & major road streams


0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80


PEC

sw a

queo

us p

hase

mg

l-1


Figure 6 Comparison of aqueous phase PECsw for diuron in the domestic use stream and the urban & major road streams

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80


PEC

sw a

queo

us p

hase

mg

l-1


Figure 7 Comparison of aqueous phase PECsw for oryzalin in the domestic use stream and the urban & major road streams


0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 70 80


PEC

sw a

queo

us p

hase

mg

l-1


Figure 8 Comparison of aqueous phase PECsw for oxadiazon in the domestic use stream and the urban & major road streams

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 70 80


PEC

sw a

queo

us p

hase

mg

l-1


Figure 9 Comparison of aqueous phase PECsw for isoxaben in the domestic use stream and the urban & major road streams


0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80


PEC

sw a

queo

us p

hase

mg

l-1

Urban streamMajor road streamDomestic ditch

Figure 10 Comparison of aqueous phase PECsw for glyphosate in the domestic use stream and the urban & major road streams

7 Conclusions

A number of data sources provided detailed information on the size of gardens associated with different housing types and the land use type within cities. Given that the data were collected in different years, in different cities and using different techniques the data compare well, giving confidence in its reliability.

Measured data were directly available, or could be used to derive parameters for: size of front garden size of patio in the back garden area of garden paved hard surface material number of houses with a garden number of houses served by a drain land use cover within the catchment runoff coefficients for hard surface materials number of households applying herbicide

Data that were absent, necessitating reasonable assumptions to be made, included the number of patios draining to a surface water drain and the actual area of each relevant hard surface type treated.

These data were used to develop a new ‘Home and Garden use’ scenario for the HardSPEC model that, because it is based on measured data and incorporates a set of justified, realistic worst-case characteristics, provides a robust tool for calculating aquatic exposure as part of the regulatory risk assessment of herbicides used in the home and garden sector

A test run of the newly developed model with six compounds indicated that peak daily aqueous phase PECsw for the six test compounds is between 6% and 88% smaller in the stream of the domestic use scenario than in that of the urban scenario. The differences appear to result mainly from the compound specific surface sorption coefficient with those compounds having the smallest concrete Kp value having the smallest differences and vice versa. Concrete surfaces comprise a major part of the domestic scenario but a much smaller component of the Urban and major road scenarios.

Note: Since completing project PS2237, further, minor amendments have been made to HardSPEC and these are reported under project PS2236.


References to published material9. This section should be used to record links (hypertext links where possible) or references to other

published material generated by, or relating to this project.Beltamn WHJ, Wieggers HJJ, de Rooy ML and Matser AM (2001). Afspoeling van amitrol, atrazin en glyfosaat vanaf een betonklinkerverharding. Alterra-rapport 319.

Gaston KJ, Warren PH, Thompson K, Smith RM. (2005) Urban domestic gardens (IV): the extent of the resource and its associated features. Biodiversity and Conservation 14: 3327-3349.

Grey CNB, Nieuwenhuijsen MJ, Golding J (2006). Use and storage of domestic pesticides in the UK. Science of the Total Environment 368: 465-470

Heather, AIJ., Shepherd, A. & Hollis, JM (1998). Losses of six herbicides from a kerb-and-gully pot road drain. Soil Survey and Land Research Centre report JF4085-1, for the Hard Surfaces Project Consortium. 28 pp

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