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For a comparison of (1) total nitrogen and total phosphorus concentrations (1985-98), (2) streamflow (1990-98), (3) load (1990-98), and (4) yield (1990-98), at the nine RIM stations, click the chart icon to the left. The data summarized in the chart can be downloaded from the Data Retrieval page. Constituent concentrations and streamflow measurements from other stations in the Chesapeake Bay Basin may be requested from USGS District Offices. |
 High resolution GIF version (65KB)
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Water-quality constituents in a river are derived from both natural and anthropogenic sources. Natural sources include such nonpoint sources as weathered rock and soils, atmospheric deposition, and decay of organic material. Water-quality constituents from these sources can enter a river in surface runoff, direct precipitation, or in ground water that discharges to a river or stream. Anthropogenic sources include point sources such as wastewater discharge from industrial and domestic treatment plants and nonpoint sources that include land affected by agriculture or urban/suburban development. Atmospheric deposition also may be considered an anthropogenic source if it has been affected by industrial or automobile emissions. Differences in concentrations and loads among the rivers can be as a result of many interrelated characteristics, including (1) land use, (2) point sources, (3) basin size, (4) runoff characteristics, (5) streambed and flood-plain characteristics, (6) streamflow velocities, and (7) hydrogeologic setting.
Nitrogen and phosphorus are of particular importance to the health of Chesapeake Bay and its estuaries because they can cause the excessive growth of undesirable plants and algae, or eutrophication . Algal blooms in the Bay deprive submerged aquatic vegetation of sunlight; algal decay consumes oxygen, which in turn has a detrimental effect on fish, crabs, molluscs, and other species. Sources of nitrogen, phosphorus, and other selected water-quality constituents are discussed below.
Nitrogen exists in natural waters as one of several different forms or a combination of these forms, depending on the source and the environmental conditions. Common forms include organic nitrogen, which can be either dissolved or particulate, and the inorganic ions ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-). The nitrogen cycle is a series of biologically-catalyzed reactions by which one form of nitrogen is transformed into another.
Certain microorganisms, such as blue-green algae and bacteria associated with the roots of leguminous plants, have the capability to transform atmospheric nitrogen (N2) to ammonium by a process commonly termed "nitrogen fixation." Other bacteria catalyze the oxidation of ammonium to nitrite and then to nitrate, a process termed "nitrification," which occurs rapidly under aerobic conditions. Under low dissolved oxygen conditions, bacteria can reduce nitrate to nitrogen gas, a process termed "denitrification," or back to nitrite and then to ammonium, termed "nitrogen reduction." Alternately, inorganic nitrogen species can be taken up by organisms and incorporated into organic material (amination), which in turn can decay and release nitrogen in the form of ammonia (deamination). All of these processes affect nitrogen transport in ground water and surface water.
Because nitrogen is so readily converted from one form to another depending on environmental conditions, identifying sources of nitrogen from analyses of different forms at a single monitoring station is difficult. However, nitrogen from specific sources enters the hydrologic cycle in characteristic forms. Sources of Total Kjeldahl nitrogen (TKN, or ammonia plus organic nitrogen) include the decay of organic material such as plant material and animal wastes and urban and industrial disposal of sewage and organic waste. Large amounts of ammonia and organic nitrogen are applied to cropland as fertilizer. Both ammonia and organic nitrogen are relatively immobile in soils and ground water because of adsorption on soil surfaces and particulate filtering, but are susceptible to nitrification under aerobic conditions.
Nitrite-plus-nitrate nitrogen can be derived from nitrification of TKN, and thus shares all the potential sources of TKN. Nitrite-plus-nitrate concentrations commonly exceed 10 mg/L in rivers affected by fertilizer application and animal wastes (Hem, 1989). Unlike ammonium ions and organic nitrogen, nitrate is highly mobile in ground water; nitrate derived from agricultural fertilizer, animal waste, or decaying plant material can infiltrate ground water, which in turn can discharge to streams. Nitrogen oxides discharged to the atmosphere by plants and the burning of fossil fuels are transformed to nitrate that is present in rain water (Drever, 1988); ammonium ions also are present in rain water (Feth, 1966). Important changes in concentration for nitrite-plus-nitrate nitrogen in streams occur through incorporation into organic matter and denitrification.
Different analyses for nitrogen are performed on water-quality samples from the River Input Monitoring stations in Virginia and Maryland: In Virginia, analyses are done for dissolved ammonia nitrogen, dissolved nitrite-plus-nitrate nitrogen, TKN, dissolved nitrogen, and particulate nitrogen. Concentration of total nitrogen is computed as the sum of either TKN and nitrite-plus-nitrate nitrogen or dissolved nitrogen and particulate nitrogen. Concentration of organic nitrogen is computed by subtracting dissolved ammonia nitrogen from TKN. In Maryland, all analyses are done independently.
Like nitrogen, phosphorus is present in natural waters in several different forms: orthophosphate, which includes species of PO43-; polyphosphates and metaphosphates, formed by the condensation of two or more orthophosphate groups; and organic phosphorus. Orthophosphate is the most thermodynamically stable and biochemically available form of phosphorus in natural waters, and microorganisms catalyze the hydrolysis of condensed phosphates to orthophosphate. However, 30 to 60 percent of the phosphorus present in many natural waters is organically bound (Snoeyink and Jenkins, 1980). Water-quality samples from this study were analyzed for dissolved orthophosphate and total phosphorus. Samples from Maryland waters also were analyzed for filtered total phosphorus.
Most phosphate salts have low solubility in water, and the positive charge on phosphate ions causes them to adsorb strongly onto particles in soils, suspended solids, and streambed sediments. Precipitation/adsorption onto sediments, and the uptake of dissolved orthophosphate by biota usually limit typical concentrations of dissolved phosphorus to no more than a few tenths of a milligram per liter (Hem, 1989). Phosphorus has low mobility in ground water, and is the limiting nutrient for vegetative growth in many surface waters.
The tendency of phosphorus to precipitate/adsorb onto soil surfaces causes a positive correlation between total phosphorus and suspended solids in many streams. Common nonpoint sources for both of these constituents is weathering of natural soils and rocks and runoff from agricultural land. Phosphate from fertilizers binds to soils, which erode during storm events adding considerable amounts of suspended phosphate to streams (Hem, 1989). Total suspended solids also are contributed to a river by soil erosion in response to lumbering and construction practices. The most important point source of phosphorus is municipal waste-water discharge, which discharges phosphate as orthophosphate and organic phosphorus.
Most dissolved silica observed in natural waters results originally from the chemical breakdown of silicate minerals in irreversible processes of weathering, and most streams in the Northeastern United States have dissolved silica concentrations less than 10 mg/L (Hem, 1989). Aquatic organisms (primarily diatoms) extract and use silica in their shells and skeletons in freshwater and in seawater.
General information: Field measurements are collected to show physical conditions at the station at the time of sample collection. Some physical conditions can greatly affect the water chemistry and may be used in conjunction with the chemical analysis to determine the upstream source of the water or to look at ongoing changes in the water.
Specific conductance (microSiemens per centimeter per degree Celsius, or uS/cm/degree C) - an indicator of the ability of water to conduct an electric current, and usually has a strong positive correlation with total dissolved solids. Diverse factors affect the specific conductance of streams, including flow conditions, bedrock geology, and contributions of dissolved solids from different point and nonpoint sources.
pH (pH units) - technically defined as the negative log of the activity of hydrogen ions in a liquid, but more commonly known as the level of acidity or alkalinity of the water, expressed on a scale from 0 (acid) to 14 (alkaline), with 7 being neutral. pH is affected by natural sources of acidity or alkalinity in the basin and point sources. Examples of natural sources affecting pH include the following: (1) weathering of limestone, which will result in a higher pH, (2) production of higher concentrations of organic acids in wetland areas, which will lower pH, and (3) the introduction of carbon dioxide into the water by algae, which will lower pH. The effect of algae is more apparent in summer months, when algal growth rates are high and discharge is low.
Dissolved oxygen (mg/L) - a measurement of the ability of water to dissolve oxygen. Dissolved oxygen concentration is normally higher during winter months because water has a greater oxygen capacity at lower temperatures. Dissolved oxygen also is high where the waters have an opportunity to be churned or mixed with air, such as in rapids. Elevated dissolved oxygen concentration in summer months can be caused by release of oxygen by photosynthetic plants. Low dissolved oxygen concentration is found in waters that are not well-mixed, such as in slow-moving streams in summer, or at depth in lakes. Algal growth and decomposition of organic material in streams and streambed sediments utilize dissolved oxygen in streams; both of these processes are more active during summer months.
Air temperature (degrees C) - used to assess quality of other measurements, and weather conditions during sample collection.
Water temperature (degrees C) - used in determination of other measurements, such as dissolved oxygen, specific conductance and pH, each of which changes with water temperature.
Barometric pressure (mm Hg) - the atmospheric pressure; used to calibrate dissolved oxygen concentration with regard to the current weather conditions.
Gage height (ft) - the water level of the stream relative to some established datum. This measurement is used to obtain instantaneous discharge at the time the water-quality sample is taken. For more information on gage height and discharge, please link to USGS Open-File Report 95-713: Measuring Streamflow in Virginia.
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