overview of continuous water-quality monitoring. purpose of monitoring define the objectives of the...

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?