A description of typical measurement parameters and their significance, as well as a look at how one state is applying the standards.

By Catherine A. Fox and Charles D. Absher

The United States Environmental Protection Agency, in conjunction with state environmental agencies, is currently required to develop total maximum daily loads (TMDLs) for in-state streams that have been placed on a state's impaired waters list, or 303(d) list. The TMDL issue is driving the Georgia Environmental Protection Division (EPD) edict that all new or renewed wastewater discharge permit applications will require a watershed assessment as part of the permitting process. Consequently, every wastewater authority in Georgia is or will soon be implementing a watershed assessment and watershed management plan.

Urban runoff contributes to poor water quality.

A typical watershed assessment includes the measurements of a number of water-quality parameters, as well as biological assessment, modeling, and often a watershed protection plan. Neither EPA nor EPD provides written guidance on the most appropriate way to design and carry out a watershed assessment in Georgia. Consequently, the resulting studies have ranged in complexity from just a few samples with a limited biological assessment (if any) to studies with numerous sample locations in which water quality is evaluated throughout the year, comprehensive biological assessment, elaborate models, comprehensive watershed protection plans, and significant public involvement. Costs of these watershed assessments over the last few years have ranged from less than $20,000 to more than $2 million. Reasons for the significant range are dependent upon watershed size, number and impacts of pollutant sources, interest from stakeholders, and available budget. Even so, the assessments of watersheds of similar type vary significantly as well. According to some at EPD, the lack of specific guidance is beneficial because it encourages innovation and creativity. To some who are carrying out watershed assessments, however, the lack of guidance and undefined minimum criteria is confusing and costly in terms of both time and resources.

This article describes the importance and interpretation of specific parameters measured in the water-quality characterization component of a typical watershed assessment. A general discussion of each parameter is followed in each section by the Georgia EPD standard for that parameter. The related article "Planning a Watershed Assessment" in the Buyers Guide 2002 issue of Stormwater described the general components of cost-effective watershed assessments; future articles will focus on other critical components of an effective watershed assessment, including sediment and biological assessments, pollutant modeling, and watershed protection plans.

Temperature

Human activity has affected the temperature of rivers and streams in many ways. One of the most significant mechanisms that increases water temperature is thermal pollution. Industries such as nuclear power plants can cause thermal impacts by discharging water used to cool machinery. Thermal impacts can also come from stormwater runoff from warmed urban surfaces, such as streets, sidewalks, and parking lots. The temperature of streams and rivers is also affected by the loss of riparian buffers, such as trees that provide shade, exposing the water to more direct sunlight. Soil erosion from overgrazing, poor farm practices, and construction can also raise water temperatures by increasing the amount of suspended solids carried by the river, making the water cloudy or turbid. Cloudy water absorbs the sun's rays, causing water temperature to rise.

Changes in water temperature have a profound effect on the stream ecosystem. As water temperature rises, the rate of photosynthesis and plant growth also increases. The additional plants eventually die and are decomposed by oxygen-consuming bacteria. Therefore, as temperature and rate of photosynthesis increase, so does the need for oxygen in the water (biochemical oxygen demand, or BOD). The metabolic rate of organisms also rises with increasing water temperature, resulting in even greater oxygen demand. The life cycles of aquatic insects tend to speed up in warmer water. Animals that feed on these insects can be negatively affected, particularly birds that depend on insects emerging at critical time periods during their migratory flights.

Most aquatic organisms have adapted to survive within a range of water temperatures. Some organisms, such as trout and stonefly nymphs, prefer cooler water, while others, such as carp and dragonfly nymphs, thrive in warmer conditions. As the temperature of a stream or river increases, warm-water organisms will replace cool-water species. Few organisms can tolerate extremes of heat or cold. Temperature also affects the sensitivity of aquatic life to toxic wastes, parasites, and disease. For example, thermal pollution might cause fish to become more vulnerable to disease, either due to the stress from rising water temperatures or the resulting decrease in dissolved oxygen.

The Georgia standard requires that discharge to a stream cannot produce a temperature change of more than 5ºF from the ambient water temperature. A maximum water temperature of 90ºF is specified for all streams.

Dissolved Oxygen

Dissolved oxygen (DO) is essential for the maintenance of healthy streams and rivers. The primary source of DO in water comes from the atmosphere through physical mixing at the air—surface water interface. Algae and rooted aquatic plants also release oxygen into streams and lakes through photosynthesis. Most aquatic plants and animals need oxygen to survive. Waters with consistently high levels of DO are generally considered to be healthy, stable ecosystems capable of supporting many different species of aquatic organisms.

DO levels in aquatic ecosystems vary significantly, depending on a number of factors. Physical influences, such as volume of discharge and water temperature, directly affect oxygen concentration; DO levels rise with increased mixing rates as well as with decreasing temperature. During dry summer periods, flow might be reduced, and air and water temperatures are often higher. Both of these factors tend to reduce DO levels. In the spring, wet weather increases flow, resulting in greater mixing and dissolution of atmospheric oxygen. Large daily fluctuations in DO are also characteristic of bodies of water with extensive plant growth. Levels rise from the morning through the afternoon as a result of photosynthesis, reaching a peak in late afternoon. Photosynthesis stops at night, but plants and animals continue to respire and consume oxygen. As a result, DO levels fall to a low point just before dawn. This phenomenon is more common in lakes and impounded rivers than in fast-flowing streams.

The main factor contributing to significant changes in DO concentrations is the buildup of organic wastes, including leaves and feces. Organic waste can enter rivers in many ways: in sewage discharges, through urban and agricultural runoff, and in the discharge of animal feeding operations and other industrial sources. A primary component of urban and agricultural runoff is fertilizers that stimulate the growth of algae and other aquatic plants. As plants die, aerobic bacteria consume oxygen in the process of decomposition. Many other kinds of bacteria also consume oxygen while decomposing sewage and other organic material.

Depletions in DO concentration cause major shifts in the kinds of aquatic organisms found in water bodies. Species that cannot tolerate low oxygen levels–mayfly and stonefly nymphs, caddis fly and beetle larvae, bass and trout–will be replaced by fewer kinds of pollution-tolerant organisms, such as worms and fly larva, carp, and catfish. Nuisance algae and anaerobic organisms might also become abundant in waters with low levels of DO.

DO levels below 5 mg/l are generally considered an indicator of poor water quality. The Georgia EPD specifies a daily average of at least 5 mg/l and an absolute minimum of 4 mg/l for waters supporting warm-water species of fish. For waters designated as trout streams, the minimum daily average is 6 mg/l, with an absolute minimum of 5 mg/l.

pH

Water (H₂O) contains both H+ (hydrogen) ions and OH- (hydroxyl) ions. The pH test measures the H+ ion concentration of liquids and substances, with resulting values reported on a scale from 0 to 14. In the US, the pH of natural water is usually between 6.5 and 8.5, although wide variations can occur.

In many parts of the country, increased amounts of nitrogen oxides (NOx) and sulfur dioxide (SO₂), primarily from automobile and coal-fired power-plant emissions, are converted to nitric acid and sulfuric acid in the atmosphere. These acids combine with moisture in the atmosphere and fall to the earth as acid rain or snow that has affected thousands of lakes, especially in the northeastern US. Geology can also affect the acidity of local water. Acid mine drainage from mining operations has impacted streams and rivers in the southeastern US as well as many other parts of the country. If limestone is present, its alkaline characteristics can act to neutralize the effect the acids have on lakes and streams. Water bodies most heavily impacted by acid rain are downwind of urban or industrial areas and do not have any limestone to buffer the impact of the acidity of the water.

Changes in the pH value of water are important to many organisms because they have adapted to life in water of a specific pH and might die if it changes even slightly, as was the case with brook trout in some streams in the Northeast. Biological communities are affected in streams that receive acid rain and acid snowmelt in the spring. Immature stages of aquatic insects and young fish are extremely sensitive to pH values at or below 5.0. Very acidic waters can also cause heavy metals, such as copper and aluminum, to be released into the water. Heavy metals accumulate on the gills of fish or cause deformities in young fish, reducing their chance of survival. At extremely high or low pH values (e.g., 9.6 or 4.5) the water becomes unsuitable for most organisms.

Georgia Water Use Classifications and Instream Water Quality Standards for designated uses require that instream pH levels be between 6.0 and 8.5.

Biochemical Oxygen Demand

BOD is a measure of the quantity of oxygen used by macroinvertebrates and bacteria in the aerobic oxidation of organic matter in streams. Typically the BOD measurement is conducted over a five-day period and termed BOD₅. Although this is not the ultimate oxygen demand the organic material might place on the water, the five-day period has become the convention for reporting BOD.

There are many natural and human sources of organic material in aquatic ecosystems. Natural sources include wetlands, bogs, and riparian vegetation, especially leaf fall. Human sources of organic material include point as well as nonpoint sources. Examples of point sources are pulp and paper mills, food-processing industries, and wastewater treatment plants. Nonpoint sources of organic material include urban runoff that carries sewage from illegal sanitary sewer connections into storm drains, pet wastes from streets and sidewalks, nutrients from lawn fertilizers, leaves, grass clippings, and paper from residential areas. Runoff from agricultural fields carries nutrients such as nitrogen and phosphates that stimulate plant growth, which in turn leads to more plant decay over time. Nutrients have been shown to be a prime contributor to high oxygen demand in many water bodies. Runoff from animal feeding operations might also carry fecal material into rivers, increasing BOD. Finally, impounded river reaches collect organic wastes from upstream areas. Once the waste materials settle in quieter waters, bacteria utilize oxygen in transforming the organic matter. Percent saturation (dissolved oxygen) values in waters with much plant growth and decay often fall below 90%.

In rivers and streams with high BOD levels, aerobic bacteria consume much of the available DO; little is available for other aquatic organisms. Organisms that are more tolerant of low oxygen levels might appear and become numerous. Examples include carp, midge larvae, and sewage worms. Organisms that are intolerant of low oxygen levels, such as the caddis fly larvae, mayfly nymphs, and stonefly nymphs, will not survive. As organic pollution increases, the ecologically stable and complex relationships present in waters with a high diversity of organisms are replaced by a low diversity of pollution-tolerant organisms.

Typically, BOD levels from 3 to 5 ppm are considered moderately clean, and levels below 3 ppm are considered very good. Georgia, however, does not have a specific instream standard for BOD.

Turbidity and Total Suspended Solids

Vegetation removal affects viable aquatic habitat.

Many factors help to define viable habitat.

Turbidity is a measure of the relative clarity of water. Turbidity increases as the result of suspended solids in water that reduce the transmission of light. The nature of suspended solids varies, depending upon the source of the material–ranging from clay, silt, and plankton to industrial wastes and sewage. The presence of suspended solids can cause color changes in water, from nearly white to red-brown or green from algal blooms. High turbidity might be caused by soil erosion, waste discharge, urban runoff, abundant bottom feeders (such as carp) that stir up bottom sediments, or algal growth.

At higher levels of turbidity, water loses its ability to support a diversity of aquatic organisms. Murkier waters become warmer as suspended particles absorb heat from sunlight, causing oxygen levels to fall. Photosynthesis decreases because less light penetrates the water, causing further decreases in oxygen content. The combination of warmer water, less light, and oxygen depletion makes it impossible for some forms of aquatic life to survive.

Suspended solids can also directly impact aquatic life in other ways. Suspended solids can clog fish gills, reduce growth rates, decrease resistance to disease, and prevent egg and larvae development. Particles of silt, clay, and organic materials settle to the bottom, especially in slower-moving rivers and streams. These settled particles can smother the eggs of fish and aquatic insects and suffocate newly hatched insect larvae. Material that settles into spaces between rocks makes these microhabitats unsuitable for mayfly nymphs, stonefly nymphs, caddis fly larvae, and other aquatic insects living there.

Turbidity measurements are reported as nephelometric turbidity units (NTUs). There is a maximum limit increase of 25 NTUs from upstream levels in the Georgia Construction Activity National Pollutant Discharge Elimination System (NPDES) discharge permit. EPA Region 4 has recently suggested using a threshold of 20 mg/l to identify waters potentially impacted by an excess of total suspended solids (TSS). However, there is no clear evidence of a dependable correlative relationship between turbidity in NTUs and TSS in mg/l.

Phosphorus

Phosphorus is usually present in natural waters as phosphate (PO₄-P). Organic phosphate is a part of living plants and animals, their byproducts, and their remains. Inorganic phosphates include the ions bonded to soil particles and phosphates present in laundry detergents.

Phosphorus is an essential nutrient needed for plant growth and a fundamental element in the metabolic reactions of plants and animals. Plant growth is limited by the amount of phosphorus available because it is usually present in very low concentrations. Any unattached or "free" phosphorus, in the form of inorganic phosphates, is rapidly taken up by algae and larger aquatic plants. Because algae require only small amounts of this nutrient to live, excess phosphorus causes extensive algal growth called "blooms." Algal blooms are a classic symptom of cultural eutrophication.

Cultural eutrophication, the human-caused enrichment of water with nutrients (usually phosphorus), is the primary cause of most eutrophication today. Natural eutrophication also takes place but is insignificant by comparison. Phosphorus taken from natural sources generally becomes trapped in bottom sediments or is rapidly taken up by aquatic plants. For example, forest fires are natural events that cause eutrophication. Lakes that receive no inputs of phosphorus from human activities age very slowly.

Phosphorus comes from several sources, including human wastes, animal wastes, industrial wastes, fertilizers, and human disturbance of the land and its vegetation. Sewage from wastewater treatment plants and septic systems is a major source of phosphorus in many aquatic ecosystems. According to EPA, sewage effluent should not contain phosphorus at levels greater than 1 mg/l, but outdated wastewater treatment plants often fail to meet this standard. Some types of industrial wastes also interfere with the removal of phosphorus during the wastewater treatment process. Storm sewers sometimes contain illegal connections to sanitary sewers, and sewage from these connections can be carried into waterways from rainfall. Phosphorus from animal wastes sometimes finds its way into rivers and lakes in the runoff from animal feeding operations.

Soil erosion from agricultural and construction activities is also a primary contributor of phosphorus to many water bodies. Fertilizers used for crops, lawns, and home gardens usually contain phosphorus, and when used in excess, the nutrient usually ends up in streams, rivers, and lakes. Draining swamps and marshes for farmland, housing, and commercial or industrial parks releases nutrients such as phosphorus that have remained dormant in years of accumulated organic deposits. In addition, drained wetlands no longer function as filters of silt and phosphorus, allowing more runoff–and phosphorus–to enter waterways.

Shallow lakes and impounded river reaches, where the water is shallow and slow-moving, are the most vulnerable to the effects of cultural eutrophication. Phosphorus stimulates the growth of algae and rooted vegetation, and the latter takes up phosphorus previously locked in bottom sediments and releases it to the water, causing further eutrophication. As eutrophication increases, swimming and boating might become impossible. Eventually, the entire lake or river stretch might fill with aquatic vegetation. The advanced stages of cultural eutrophication can produce anaerobic conditions in which oxygen in the water is completely depleted. These conditions occur near the bottom of a lake or impounded river stretch and produce gases such as hydrogen sulfide, unmistakable for its rotten-egg smell.

Water bodies with total phosphorus present at levels above 0.1 mg/l might be considered at risk from cultural eutrophication. Georgia has no instream standard for phosphorus.

Nitrogen

Nitrogen is an element needed by all living plants and animals to make protein. In aquatic ecosystems, nitrogen is present in many forms. In nature, nitrogen is a much more abundant nutrient than phosphorus. It is more commonly found in its molecular form (N₂), which makes up 79% of the air we breathe. This form is useless for most aquatic plant growth. However, blue-green algae, the primary algae of algal blooms, are able to use N₂ and convert it into other forms of nitrogen, specifically ammonia (NH₃) and nitrate (NO^-₃), which plants can take up through their roots and use for growth. Animals obtain the nitrogen they need by either eating aquatic plants or eating other aquatic organisms that feed upon the plants. As aquatic plants and animals die, bacteria break down large protein molecules into ammonia, which is then oxidized by other bacteria to form nitrites (NO^-₂) and nitrates (NO^-₃).

Excretions of aquatic organisms are very rich in ammonia, although the amount of nitrogen they add to waters is usually small. In areas where ducks and geese are plentiful, however, they contribute a heavy load of nitrogen from excrement. Through decomposition of dead plants and animals and the excretions of living animals, nitrogen that was previously "locked up" is released. Some bacteria can also transform nitrates (NO^-₃) into free molecular nitrogen (N₂). The nitrogen cycle begins again if this free molecular nitrogen is converted by blue-green algae into ammonia and nitrates.

Because nitrogen, in the form of ammonia and nitrates, acts as a plant nutrient, it also causes eutrophication. As described in the previous section on phosphorus, eutrophication promotes plant growth and decay, which in turn increases BOD. Nitrogen, unlike phosphorus, however, rarely limits plant growth, so plants are not as sensitive to increases in ammonia and nitrate levels.

Sewage is the main source of nitrates added by humans to rivers. Sewage enters waterways in inadequately treated wastewater from sewage treatment plants, in the effluent from illegal sanitary sewer connections, and from poorly functioning septic systems. Septic systems, more common in rural areas, generally treat waste from a single household. If these systems are located too close to the water table or if the systems are not emptied periodically, nutrients and bacteria can get into the drinking-water supply from a nearby well or can travel through the ground or through surface runoff to nearby streams and lakes. Water containing high nitrate levels, if used to make infant milk formula, can cause a serious condition called methemoglobinemia. This condition prevents the baby's blood from carrying oxygen; hence the nickname "blue baby syndrome."

Two other important sources of nitrates in water are fertilizers and runoff from cattle feedlots, dairies, and barnyards. High nitrate levels have been found in groundwater beneath croplands because of excessive fertilizer use, especially in heavily irrigated areas with sandy soils. Stormwater runoff can carry nitrate-containing fertilizers from farms and lawns into waterways. Similarly, places where animals are concentrated, such as feedlots and dairies, produce large amounts of waste rich in ammonia and nitrates. If not properly contained and treated, bacteria and nutrients can seep into groundwater or be transported to surface waters. The resulting eutrophication can limit organism diversity, recreational opportunities, and property values.

Typically, concentrations of total nitrate nitrogen above 10 mg/l, nitrite above 0.1 mg/l, ammonia nitrogen above 2 mg/l, and total Kjeldahl nitrogen above 2 mg/l are a concern and suggest that actions should be taken to identify sources and limit inputs of nitrogen in the ecosystem. There are no Georgia instream standards for nitrogen components, but there is the nitrate/nitrite standard of 10 mg/l and 0.1 mg/l, respectively, applied to drinking-water supplies.

Fecal Coliform

Controlling construction runoff is critical.

An in-situ meter is essential for assessments.

Fecal coliform bacteria are found in the feces of humans and other warm-blooded animals. These bacteria can enter rivers through direct discharge from mammals and birds, from agricultural and storm runoff carrying animal waste, and from human sewage discharged into the water. Fecal coliform bacteria by themselves are not pathogenic. Pathogenic organisms that cause diseases and illnesses include not only bacteria but also viruses and parasites. Fecal coliform bacteria occur naturally in the human digestive tract and aid in the digestion of food. In infected individuals, pathogenic organisms are found along with fecal coliform bacteria.

Pathogens are relatively scarce in water, making them difficult and time-consuming to monitor. Instead, fecal coliform levels are monitored because of the correlation between fecal coliform counts and the presence of pathogenic organisms. If an analysis indicates the presence of fecal coliform counts are higher than 200 colonies per 100 ml of stream water sampled, the potential for pathogenic organisms to be present also exists. A person swimming in such waters has a greater chance of getting sick from swallowing disease-causing organisms or from pathogens entering the body through cuts in the skin, nose, mouth, or ears. Diseases and illness, such as typhoid fever, gastroenteritis, dysentery, and ear infections, can be contracted in waters with high fecal coliform counts.

Cities and small towns sometimes contribute human wastes to local rivers through their sewer systems. A sewer system is a network of underground pipes that carries wastewater. In a separate sewer system, sanitary wastes flow through sanitary sewers and are treated at a wastewater treatment plant, and storm sewers carry stormwater runoff from streets and discharge the untreated stormwater directly into streams and rivers. Rainfall can wash animal wastes produced by pets, birds, squirrels, and so on from lawns, sidewalks, and streets into streams. Rainfall can also flush fecal coliform from sanitary sewer overflows into streams. In a combined sewer system, both sanitary wastes and storm runoff are treated at the wastewater treatment plant.

Much controversy currently exists at the national, state, and local levels regarding the usefulness of the fecal coliform indicator. The importance of this parameter in Georgia is especially significant because 92% of the state's 303(d)-listed water bodies are impaired by exceedances of the fecal coliform standard. Discussions are underway to resolve this issue; however, no clear approach has yet been identified that will satisfy the many stakeholders involved in this controversy. In the meantime, Georgia has in place several sets of standards depending on the water-use classification of the water body in question.

The fecal coliform standard applied to drinking water and fishing classification streams is as follows: "For the months of May through October, when water contact recreation activities are expected to occur, fecal coliform is not to exceed a geometric mean of 200 per 100 ml based on at least four samples collected from a given sampling site over a 30-day period at intervals not less than 24 hours. Should water-quality and sanitary studies show fecal coliform levels from nonhuman sources exceed 200/100 ml (geometric mean) occasionally, then the allowable geometric mean fecal coliform shall not exceed 300 per 100 ml in lakes and reservoirs and 500 per 100 ml in free-flowing freshwater streams. For the months of November through April, fecal coliform is not to exceed a geometric mean of 1,000 per 100 ml based on at least four samples collected from a given sampling site over a 30-day period at intervals not less than 24 hours and is not to exceed a maximum of 4,000 per 100 ml for any sample. The State does not encourage swimming in surface waters since a number of factors which are beyond the control of any State regulatory agency contribute to elevated levels of fecal coliform."

A similar standard exists for waters classified as recreational, with coastal waters specified a maximum geometric mean of 100 per 100 ml. Georgia allows a higher standard for all classifications if the source can be proven nonhuman.

Toxic Metals

Volcanic eruptions, weathering of rock, and other natural processes continually introduce and cycle metals in the environment. This geological weathering is responsible for the background levels of metals found in rivers and lakes. Natural processes and cycles are often disrupted by human activity such as mining (e.g., lead, silver, copper, and iron ore) and manufacturing processes that redistribute and concentrate metals in the environment. Metals are often found in the effluent of various manufacturing processes, including lead and nickel from battery manufacturing, copper from the textile industry, silver from photographic film production, and iron ore from steel production. Other point sources, such as sewage effluent, might contain elevated levels of copper, lead, zinc, and cadmium. Some of this increase has been linked to corrosion within the wastewater supply pipes.

Nonpoint sources of pollution include both urban and rural runoff. Urban stormwater runoff carries increased metal loadings, especially during the initial "first flush" phase of the rain event. Stormwater carries lead deposited on streets and parking lots from car exhaust, oil and grease, zinc from motor oil and grease, and copper worn from metal plating and brake linings. In rural areas, sediments eroding from croplands carry cadmium and even uranium, which are both found in some phosphate fertilizers. Herbicides used to control weeds might also contain arsenic. In addition, metals used in such common products as cars eventually end up in landfills, or their byproducts can be transported via stormwater to a nearby water body.

Many metals are essential for plant and animal growth and metabolism. Nickel, zinc, and copper are considered essential elements. At excessive levels, essential trace metals become toxic to invertebrates and fish. Often the difference between nontoxic and toxic levels is minute. Nonessential elements, such as cadmium, mercury, and lead, are toxic even at very low levels. Toxicity refers to the potential harmful effects, both lethal and nonlethal, of a chemical upon a living organism. Potential effects include the inability to reproduce, behavioral changes, and changes in growth and development. It is often difficult to differentiate the many interconnected effects of toxic metals. For example, a fish that is stressed by accumulation of metals might become physically less able to avoid predation.

The toxicity of heavy metals to aquatic organisms depends upon many factors, including the bioavailability of metals to organisms. Organisms take up metals through ingestion of food, through adsorption onto membranes (gills), and by transport through the skin. Bioavailability, in turn, is influenced by water hardness, pH, life cycle, age of the organism, and water temperature. With increasing water hardness, the toxicity of metals decreases as they are adsorbed onto insoluble carbonate compounds. A lowering of the pH increases the solubility of metals in solution. Below a pH of 5.5, aluminum and mercury levels can be a threat to aquatic life. Concentrations of metals, such as mercury, are often higher in older organisms. An increase in water temperature increases metabolism and quickens the intake of metals as well. Metals are adsorbed onto organic material and so are found concentrated in bottom sediments. Organisms that inhabit metal-laden sediment (e.g., tubifex worms) exhibit high levels of metals. People who eat bottom-feeding fish, such as carp and catfish, on a frequent basis might be at increased health risk.

EPA has defined acute and chronic water-quality standards for many individual metals. These standards have generally been adopted by all states, unless they choose to be more protective. For example, Georgia EPD has set for freshwater ecosystems an acute maximum standard of 64 µg/l and a chronic maximum standard of 58 µg/l for zinc, a typical metal found in Georgia's streams. Acute and chronic standards are also specified for many other metals, including arsenic, copper, lead, and mercury. Acute levels are those at which aquatic life will suffer deleterious effects after a short period of exposure, typically one hour. Chronic levels are those in which aquatic life will suffer deleterious effects after prolonged exposure, typically four days.

The concentration of total metals in stream water can also be estimated as specific conductance (conductivity). Specific conductivity is a numerical expression of water's ability to conduct an electrical current. It is typically measured in microsiemens per centimeter (µS/cm). Values of high specific conductance reflect the presence of high concentrations of total dissolved solids or potentially dissolved metals. Specific conductance in natural surface waters has been found to range from 50 to 150 µS/cm (0.05 to 0.15 mS/cm). The common standard used for freshwater monitoring in Georgia is 147 µS/cm (0.147 mS/cm).

Toxic Organic Compounds

As with metals, organic chemical contaminants, such as pesticides, polycyclic aromatic hydrocarbons, and other chlorinated compounds, can interfere with normal biological processes or even be lethal to aquatic organisms in certain conditions. These chemicals are produced and released into the environment as point-source pollution through inadequate treatment of byproducts of industrial processes and household products (e.g., bleach and drain cleaners), or as nonpoint-source pollution through street runoff, atmospheric deposition, and herbicide and insecticide runoff from croplands and residential areas.

Some toxins bind to soil particles and are easily washed into water bodies, where they cause an overall decline in numbers and types of aquatic organisms found there. Some types of midges and aquatic worms are more tolerant than other organisms, so their numbers might increase although overall stream diversity will go down. Toxic organic compounds can enter the food chain through organisms that process sediment. Other animals eat these organisms, which are in turn eaten by larger animals higher in the food chain in a process called bioaccumulation. In larger fish, toxins can cause lesions and deformities.

Many of these contaminants are difficult to monitor directly and so require more expensive and time-consuming laboratory tests of water samples. Often a priority pollutant scan will be carried out to determine if toxic organic chemicals are present. This analysis measures numerous compounds, including nearly 35 volatile organics, 60 chlorinated aromatics, 26 pesticides, 10 herbicides, and 15 metals. If the results indicate that one or more of the chemicals are present at levels of concern, additional samples will likely be taken in hopes of effectively characterizing the extent and source of the pollutant into the system. In addition, macroinvertebrates and fish samples may be collected and analyzed to determine if levels in their tissue are a concern to human health. Laboratory toxicity testing of sediments and water using a variety of sensitive aquatic species may also be carried out to evaluate overall impacts to the aquatic ecosystem and human health.

EPA has defined acute and chronic water-quality standards for many individual organic contaminants. Unlike metals, there is no simple method to estimate the potential impacts of organic contaminants in an aquatic ecosystem; samples must be evaluated as described above.

Summary

Assessing water quality is not a simple matter of taking a sample and testing it in the laboratory. The parameters described here, summarized in Table 1, are major ones generally considered in assessments. There is a danger, however, in applying too much importance to a single measurement of water quality. Trends in water-quality data are more significant than single spikes. The overall characterization of a watershed should not depend on one parameter or one measurement. A sound characterization should gather evidence from a variety of land uses, runoff conditions, and environmental components. Even if the parameters described here are all below accepted standards, this does not definitively indicate a healthy system. Clean water does not necessarily translate to a viable aquatic habitat. In a future article we will address the sediment and biological components of a watershed assessment that serve to round out and complete the characterization process.

Catherine A. Fox is a senior environmental scientist with the Georgia Tech Research Institute and principal of FOX Environmental in Decatur, GA. Charles D. Absher, P.E., is a senior engineer with Integrated Science & Engineering in Griffin, GA.

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