overview of continuous water-quality monitoring. purpose of monitoring define the objectives of the...
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Overview of Continuous Water-Quality MonitoringOverview of Continuous Water-Quality Monitoring
Purpose of MonitoringPurpose of Monitoring
Define the objectives of the water quality monitoring project1. Environmental impacts of effluent
2. Contaminant alerts
3. Plume tracking
4. Trends
How will data be used ?How will data be used ?
Investigate variations in water quality1. Event
2. Diurnal
3. Monthly
4. Seasonal
5. Annual Evaluate loads (requires flows) Regulations (daily mean, max, min) Threshold warnings Development of surrogate relations
Design of Monitoring PlanDesign of Monitoring Plan
Data requirements
1. Period and duration1. Seasonal
2. Short term
3. Long-term
2. Frequency of data collection1. Continuous or discrete
2. 15 minute, hourly, daily, monthly, etc…
3. Sensor selection
Water-Quality ParametersWater-Quality Parameters
Common parameters measured: Temperature Specific conductance Salinity (based on specific conductance) pH Dissolved oxygen Turbidity
Typical Probe SpecificationsTypical Probe Specifications
Maximum depth: 60 m Temperature: -5 to 50o Celsius Specific conductance: 0 to 100 mS/cm Salinity: 0 to 80 ppt pH: 0 – 14 pH units Dissolved oxygen: 0 to 50 mg/L, 0 to
500 % saturation Turbidity: 0 to 1,000 NTU
YSI
Water-Quality SensorsWater-Quality Sensors
Chlorophyll-a (algae)
Temperature and Specific Conductance
Turbidity
pH
OpticalDissolved Oxygen
Clark CellDissolved Oxygen
Other SensorsOther Sensors
Troll
Hydrolab
Other SensorsOther Sensors
TemperatureTemperature
Thermistor Resistance changes with temperature Resistance converted to temperature
using algorithm Common unit: degrees Celsius
Specific ConductanceSpecific Conductance
Measure of the water’s ability to conduct electrical current
Electrodes must be submerged in water Approximate measure of the amount of
dissolved solids or ions in water Specific conductance is conductance
“normalized” to 25 degrees C Common unit: uS/cm (microSiemens per
centimeter), also umhos/cm (same units)
SalinitySalinity
Not measured directly Computed parameter based on
conductivity and temperature Essentially measuring the amount of
chloride in water Common unit: ppt (parts per thousand)
pHpH Measure of acid/base characteristics pH 7.0 = neutral pH > 7.0 = alkaline/basic pH < 7.0 = acidic Measures differential of hydrogen ions (H+)
inside/outside of electrode Common unit: standard pH units
Dissolved OxygenDissolved Oxygen
2 major types Rapid pulse Clark cell Optical
Common units: mg/L and % saturation
Advantages of Optical SensorsAdvantages of Optical Sensors
Less susceptible to FOULING Less susceptible to CALIBRATION
DRIFT Sensors require fewer site visits Still need routine cleaning
Advantages of Optical Sensors, cont.Advantages of Optical Sensors, cont.
More rugged Greater range of operation More accurate readings at low DO No need for stirring Not strongly affected by
temperature
TurbidityTurbidity
Measure of water clarity Light is emitted, scatters off particles Amount of light scattered at 90 degrees
is measured Common units (depends on probe):
NTU (nephelometric turbidity units) FNU (formazin turbidity units)
TurbidityTurbidity
Detector measures how much light is scattered
at 90 degrees
Light source
Sample
DetectorPhoto courtesy of Sontek YSI Inc.
Installation type
1. Flow through
2. In situ1. internal logger and power
2. external logger and power
Design of Monitoring PlanDesign of Monitoring Plan
Flow through systemFlow through system
Water from river outlet
Flow throughFlow through
Advantages1. Secure
2. Reduced fouling
3. Real-time data access Disadvantages
1. Requires AC electric service
2. More maintenance
3. Results can be less accurate (turbidity)
In situ (external logger) In situ (external logger)
In situ (external logger)In situ (external logger)
Advantages1. Data are secure
2. Real-time data access
3. Instream monitoring often yields more accurate results
4. No AC requirement permits remote sites Disadvantages
1. Sonde and probes are vulnerable to vandalism and loss
2. Probes are subject to fouling and damage from debris
In situ (internal logger) In situ (internal logger)
In situ (internal logger)In situ (internal logger)
Advantages1. Remote locations possible
2. Instream monitoring often yields more accurate results
3. Less maintenance Disadvantages
1. Telemetry not an option
2. Sonde, probes, and data are vulnerable to vandalism and loss
3. Probes are subject to fouling and damage from debris
Continuous Water Quality Monitoring
Continuous Water Quality Monitoring
Advantages Needed in rapidly
changing systems Provides better
understanding of interaction between constituents
Provides better understanding of transport processes
Disadvantages Equipment costs are
greater Operation and
maintenance costs are greater
Vulnerable to damage and/or loss
Relations Between ParametersDO and pH
Relations Between ParametersDO and pH
DO and pH track together
Diurnal Pattern Why?
Aquatic organisms produce CO2 at night combining with H20 to form H2CO3 (carbonic acid) causing pH to go down.
Relations Between ParametersTurbidity –vs- Discharge
Relations Between ParametersTurbidity –vs- Discharge
Discrete vs Continuous Monitoring
Discrete vs Continuous Monitoring
Other Surrogate PossibilitiesOther Surrogate PossibilitiesContinuous Parameter(s) Surrogate Constituent
Specific Conductance TDS, Total Nitrogen
Turbidity Suspended Sediment, Total Phosphorous
Turbidity + Temperature Bacteria
Relations are developed using discrete samples and linear regression
Regression model used to synthesize continuous record of target parameters that are difficult to monitor.
Parameter -vs- surrogate relations are not universal but site specific
ApplicationsApplications
Continuous monitoring the constituent or its surrogate to aid in identifying occurrence and duration of water-quality parameters that exceed regulatory limits.
Relation between SC and TNRelation between SC and TN
100 150 200 250 300 350 400 450 500 550
-0.5
0
0.5
1
1.5
2
f(x) = 0.00407283582457901 x − 0.636712862716251R² = 0.902010678647625
ln(TN)
Linear (ln(TN))
SC (uS/cm)
TN
(m
g/L
)
Proposed Regulatory Limit = 1.0 mg/L
Easy to monitor
Difficult to monitor
Applications (cont)Applications (cont)
Identify and optimize periods for sample collection
Quantify constituent loads (volume/time)
Familiarity with the site and data will lead to a better understanding of physical processes and interactions between constituents
Questions?Questions?