Source Water Quality

Source-water quality determines much of the work a drinking-water treatment plant must perform. Operators need to understand the normal characteristics of each source, recognize changes quickly, and connect raw-water conditions to treatment performance.

Surface water and groundwater can have very different physical, chemical, and microbiological characteristics. Even one source can change significantly with weather, season, pumping conditions, reservoir conditions, or land-use events.

What Is Source-Water Quality?

Source-water quality describes the physical, chemical, and biological characteristics of water before treatment.

Important source-water indicators can include:

• turbidity;
• temperature;
• pH;
• alkalinity;
• hardness;
• conductivity;
• organic matter;
• iron and manganese;
• microorganisms;
• nutrients.

Know the Normal Baseline

Operators should know the normal range and seasonal pattern of their source.

A useful baseline can include:

• typical values;
• seasonal highs and lows;
• storm response;
• drought response;
• normal treatment demand.

Surface Water Versus Groundwater

Surface water is generally more exposed to:

• weather;
• runoff;
• microorganisms;
• organic matter;
• algae;
• rapid upstream changes.

Groundwater often has more stable short-term quality but can contain higher concentrations of dissolved minerals.

Typical Surface-Water Challenges

Surface-water sources commonly present challenges involving:

• turbidity;
• suspended solids;
• natural organic matter;
• microbial contamination;
• algae;
• seasonal temperature change.

Typical Groundwater Challenges

Groundwater sources may contain elevated:

• hardness;
• alkalinity;
• iron;
• manganese;
• dissolved minerals;
• conductivity.

Turbidity

Source-water turbidity can result from:

• soil;
• clay;
• silt;
• organic particles;
• microorganisms;
• algal material.

Surface-Water Turbidity

Surface-water turbidity can rise rapidly after:

• heavy rainfall;
• snowmelt;
• erosion;
• construction;
• reservoir disturbance.

High Turbidity Changes Treatment Demand

Higher raw-water turbidity can increase:

• coagulant demand;
• sludge production;
• clarifier loading;
• filter loading;
• backwash frequency.

Groundwater Turbidity

Groundwater usually has lower turbidity than surface water, but a sudden increase may indicate:

• sand production;
• well construction problems;
• rapid pumping changes;
• surface influence;
• sample or instrument problems.

Natural Organic Matter

Natural organic matter enters source water from:

• soil;
• vegetation;
• wetlands;
• decaying biological material;
• watershed runoff.

Organic Matter and Treatment

Higher organic matter can increase:

• coagulant demand;
• chlorine demand;
• color;
• disinfection byproduct precursor load.

Storms Can Increase Organic Matter

Runoff can transport organic material from the watershed into rivers and reservoirs.

Operators may observe increasing organic matter along with:

• turbidity;
• color;
• chlorine demand.

pH

Raw-water pH affects treatment chemistry.

Changes in source-water pH can affect:

• coagulation;
• chemical solubility;
• disinfection;
• corrosion-control requirements.

Alkalinity

Source-water alkalinity helps determine how strongly the water resists pH change.

Low alkalinity can make treatment pH more sensitive to chemical addition.

Coagulation and Alkalinity

Some coagulants consume alkalinity.

If raw-water alkalinity is low, treatment may produce a larger pH decrease than expected.

Hardness

Hardness is mainly caused by dissolved calcium and magnesium.

Groundwater commonly develops greater hardness through prolonged contact with mineral formations.

Hardness Can Vary by Source

Systems using multiple wells or a combination of surface water and groundwater may experience substantial changes in hardness when the source blend changes.

Conductivity

Conductivity provides a rapid indication of dissolved ionic material.

Source-water conductivity may change because of:

• source switching;
• groundwater contribution;
• road salt;
• industrial discharge;
• drought concentration effects.

Use Conductivity as a Trend Indicator

A conductivity change is useful evidence that source conditions changed, but conductivity alone does not identify the specific contaminant.

Iron

Iron commonly occurs in groundwater under reducing conditions.

When dissolved iron is oxidized, it can form visible particles.

Iron can cause:

• red or brown color;
• staining;
• sediment;
• treatment deposits.

Manganese

Manganese can also occur in groundwater and low-oxygen reservoir water.

It can contribute to:

• dark staining;
• black particles;
• deposits;
• consumer complaints.

Reservoir Iron and Manganese

During thermal stratification, deep reservoir water may become low in dissolved oxygen.

Reducing conditions can increase soluble iron and manganese near the bottom.

Reservoir Intake Depth Matters

If a plant can withdraw water from different depths, intake selection may affect:

• iron;
• manganese;
• temperature;
• algae;
• dissolved oxygen.

Temperature

Source-water temperature affects:

• reaction rates;
• coagulation;
• settling;
• disinfection;
• biological activity.

Seasonal Temperature Change

Surface-water temperature often changes substantially through the year.

Groundwater temperature is usually more stable.

Cold Source Water

Cold water can make treatment more difficult because:

• chemical reactions slow;
• floc can form more slowly;
• settling can be less effective.

Warm Source Water

Warm water can increase:

• biological activity;
• algal growth;
• disinfectant decay;
• taste and odor concerns.

Microbiological Source Quality

Source water can contain:

• bacteria;
• viruses;
• protozoa;
• other microorganisms.

Surface-Water Microbial Risk

Surface water is generally more exposed to microbial contamination from:

• wildlife;
• livestock;
• septic systems;
• wastewater discharges;
• storm runoff.

Groundwater Microbial Risk

Groundwater can also become microbiologically contaminated through:

• poor well construction;
• flooding;
• surface influence;
• fractures;
• contaminated recharge.

Clear Water Is Not Proof of Microbiological Safety

Microorganisms may be present even when water appears clear and has no unusual taste or odor.

Algae

Algal growth in lakes and reservoirs can affect source water by changing:

• pH;
• taste and odor;
• organic matter;
• filter loading;
• disinfectant demand.

Nutrients

Nitrogen and phosphorus can support algal and biological growth.

Source-water nutrient levels may increase because of:

• agricultural runoff;
• wastewater discharges;
• urban runoff.

Taste and Odor

Raw-water taste and odor problems can be associated with:

• algae;
• decaying organic matter;
• industrial contamination;
• reducing conditions.

Color

Source-water color can result from:

• natural organic matter;
• iron;
• manganese;
• industrial influence.

Dissolved Oxygen

Dissolved oxygen can provide information about source-water conditions.

Low DO can be associated with:

• organic decomposition;
• reservoir stratification;
• reducing conditions.

Source Water Changes During Storms

Storms can cause simultaneous increases in:

• flow;
• turbidity;
• organic matter;
• microorganisms;
• nutrients;
• debris.

Storm Response Can Be Delayed

The water reaching an intake may respond hours or longer after rainfall depending on:

• watershed size;
• distance;
• stream velocity;
• reservoir storage.

Watch the Rate of Change

A rapid raw-water change may require faster treatment adjustment than a gradual seasonal trend.

Drought Effects

Drought can cause:

• lower streamflow;
• lower reservoir levels;
• lower groundwater levels;
• higher temperature;
• greater concentration of some dissolved constituents.

Low River Flow

During low flow, an upstream discharge may represent a larger fraction of total river flow.

This can increase its influence on source-water quality.

Flood Effects

Flooding can introduce:

• high turbidity;
• microorganisms;
• fuel;
• chemicals;
• debris.

Source-Water Quality and Treatment

Treatment should respond to actual source-water conditions.

A fixed chemical setting may become inappropriate when:

• turbidity changes;
• organic matter changes;
• temperature changes;
• alkalinity changes;
• source blend changes.

Raw-Water Turbidity and Coagulation

When raw-water turbidity changes, operators should review:

• coagulant dose;
• coagulation pH;
• floc formation;
• settled-water quality;
• filter loading.

Raw Organic Matter and Disinfection

Increasing organic matter can increase disinfectant demand.

It can also increase the amount of material available to form disinfection byproducts.

Raw-Water Alkalinity and pH Control

A decrease in raw-water alkalinity can make treatment pH more sensitive to coagulant or acid addition.

Iron and Manganese Treatment Demand

If raw iron or manganese increases, operators may need to review:

• oxidation conditions;
• chemical dose;
• filter performance;
• source or intake selection.

Source Blending

Water from multiple sources may be blended before or during treatment.

Blending can change:

• hardness;
• alkalinity;
• pH;
• conductivity;
• organic matter;
• temperature;
• disinfectant demand.

Calculate Source Blend Percentage

A simple blend percentage can be calculated as:

Source Fraction, % = Source Flow ÷ Total Flow × 100

Blend Example

A plant receives:

• 2 MGD from Source A;
• 3 MGD from Source B.

Total flow:

2 + 3 = 5 MGD

Source A fraction:

2 ÷ 5 × 100 = 40%

Source B provides 60 percent of the blend.

Simple Blended Concentration

For a conservative constituent that mixes without significant reaction, a simplified concentration relationship is:

Blended Concentration = (Q1C1 + Q2C2) ÷ (Q1 + Q2)

Blend Concentration Example

Source A:

• 2 MGD;
• hardness = 100 mg/L.

Source B:

• 3 MGD;
• hardness = 200 mg/L.

Blended hardness:

((2 × 100) + (3 × 200)) ÷ 5

(200 + 600) ÷ 5 = 160 mg/L

Not Every Parameter Blends Simply

Parameters affected by chemical reactions, equilibrium, biological activity, or nonlinear relationships may not behave as a simple weighted average.

pH is an important example that should not normally be blended by simple arithmetic averaging.

Source Switching

When changing sources, operators should review:

• raw-water quality;
• chemical demand;
• treatment configuration;
• finished-water response.

Do Not Wait for Finished Water to Change

If the incoming source is known to be changing, operators can begin monitoring treatment response before the change reaches finished water.

Source-Water Sampling

Raw-water samples should be representative of the actual source entering treatment.

Operators should understand whether the sample represents:

• an individual well;
• a river intake;
• a reservoir depth;
• a blended raw-water stream.

Individual Well Sampling

Sampling individual wells is important when wells have different:

• iron;
• manganese;
• hardness;
• conductivity;
• microbiological characteristics.

Reservoir Depth Sampling

Sampling different reservoir depths can help identify:

• temperature stratification;
• low dissolved oxygen;
• iron and manganese;
• algae.

Trend Raw-Water Quality

Useful trends include:

• turbidity;
• temperature;
• pH;
• alkalinity;
• conductivity;
• organic matter;
• iron and manganese.

Connect Raw-Water Trends to Plant Data

Compare source trends with:

• coagulant dose;
• settled-water turbidity;
• filter performance;
• chlorine dose;
• chlorine residual;
• finished-water pH.

Example: Storm Turbidity Increase

If raw turbidity rises after rainfall while filter effluent remains stable, treatment is successfully handling the increased load.

Example: Raw Turbidity Stable, Filter Turbidity Rises

This points more strongly toward a treatment-process or filter problem than a source-water event.

Example: Chlorine Demand Increases

If chlorine demand rises, review:

• raw organic matter;
• ammonia where relevant;
• temperature;
• source blend;
• chemical strength.

Example: Groundwater Conductivity Changes

Review:

• which well is operating;
• pumping rate;
• source blending;
• road-salt or contamination influence;
• instrument calibration.

Example: Iron Suddenly Increases

Review:

• source well or intake depth;
• reservoir dissolved oxygen;
• well pumping conditions;
• oxidation system performance.

Verify Unexpected Results

Before making large treatment changes, confirm unusual source-water results using:

• repeat sample;
• instrument check;
• laboratory analysis;
• related parameters.

Multiple Indicators Strengthen the Diagnosis

A real source-water event becomes more likely when several related indicators change together.

For example:

• rainfall plus turbidity increase;
• reservoir turnover plus lower DO and higher manganese;
• source switching plus conductivity and hardness change.

Source-Water Quality and Treatment Optimization

Good source monitoring helps operators avoid both:

• under-treatment;
• unnecessary chemical overfeed.

Common Source-Water Quality Mistakes

• Assuming raw-water quality remains constant.
• Using the same treatment settings after a major source change without verification.
• Ignoring storm travel time.
• Ignoring seasonal temperature effects.
• Assuming clear groundwater has no water-quality problems.
• Ignoring source blending.
• Using conductivity to identify a specific contaminant.
• Ignoring reservoir depth differences.
• Reacting to one unusual measurement without verification.
• Looking at plant performance without reviewing raw-water conditions.

A Practical Surface-Water Quality Review

1. Review current weather and watershed conditions.
2. Review raw-water turbidity.
3. Review temperature and pH.
4. Review alkalinity and organic-matter indicators.
5. Review algae or taste and odor conditions.
6. Review intake depth or river conditions.
7. Compare current results with seasonal baseline.
8. Review corresponding treatment response.

A Practical Groundwater Quality Review

1. Confirm which well is operating.
2. Review turbidity.
3. Review pH and alkalinity.
4. Review hardness and conductivity.
5. Review iron and manganese.
6. Review microbiological results.
7. Compare with pumping and water-level conditions.
8. Compare current results with historical well data.

A Practical Source-Change Review

1. Identify the source or blend change.
2. Compare old and new raw-water characteristics.
3. Review chemical-feed requirements.
4. Review coagulation and filtration performance.
5. Review disinfection demand.
6. Trend finished-water response.
7. Document treatment adjustments and results.

What to Remember for the Exam

• Source-water quality includes physical, chemical, and biological characteristics before treatment.
• Surface water generally changes more rapidly than groundwater.
• Surface-water challenges commonly include turbidity, organic matter, microorganisms, algae, and seasonal variability.
• Groundwater commonly contains higher concentrations of dissolved hardness, alkalinity, iron, manganese, and other minerals.
• High raw-water turbidity can increase coagulation, sludge, filtration, and backwash demands.
• Natural organic matter can increase coagulant demand, chlorine demand, color, and disinfection byproduct precursor load.
• Source-water pH and alkalinity strongly influence treatment chemistry.
• Conductivity is useful for detecting source changes but does not identify a specific contaminant.
• Low dissolved oxygen in deep reservoir water can increase soluble iron and manganese.
• Source-water temperature affects reaction rates, coagulation, settling, disinfection, and biological activity.
• Clear groundwater is not automatically free of microbiological or dissolved-chemical problems.
• Storms can cause rapid changes in turbidity, organic matter, nutrients, microorganisms, and debris.
• Drought and low-flow conditions can also change source-water quality.
• Source blending can change hardness, alkalinity, conductivity, organic matter, temperature, and chemical demand.
• A simple blend fraction equals source flow divided by total flow.
• Conservative constituents can often be estimated with a flow-weighted concentration calculation.
• pH should not normally be treated as a simple arithmetic blend.
• Unexpected raw-water results should be verified before major treatment changes are made.
• Raw-water trends should be compared with treatment and finished-water trends.
• Good source-water management means anticipating change and adjusting treatment based on verified source conditions.