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for further study of the effects of landmanagement practices in clay-type soils on ground-water quality. In particular, the hydrologists will be interested to see what changes over time occur in water quality in the mixed land-use basin as it is converted to best-management practices.
standard-management practices field. In contrast, this same field has more plowed ground in the barren season. Plowing the ground increases the surface runoff and promotes infiltration by causing water to be trapped. In spite of the fact that the runoff is greater on the average in the best-management practices field during the barren season, sediment yields during this period are still lower than those observed for the standard-management practices field. This difference indicates that the effect of best-management practices in reducing sediment losses is significant enough to compensate for the additional runoff.
The effects of land-management practices on ground-water quality are not as apparent as the effects on surface water. Mean concentrations of total nitrate plus nitrite in the upper unsaturated zone (3 feet below land surface) are higher in the field having standardmanagement practices where concentrations averaged 1.7 mg/L compared with 1.0 mg/L in the field having bestmanagement practices. Water samples collected in the unsaturated zone 6 feet below land surface and from ground water from 10 to 15 feet below land surface, however, have concentrations slightly higher in the best-management practices field than those measured in the standard-practices field. Levels of pesticide residues measured in soil samples tend to be higher in the bestmanagement practices field compared with the standard-management practices field.
USGS hydrologists are continuing their sampling of surface and ground water in the four study areas. The project results thus far support the need
Relations Between Land Use and Nitrate Concentrations in Shallow Ground Water, Delmarva Peninsula
By Pixie A. Hamilton and Robert J. Shedlock
The USGS began to develop a National Water-Quality Assessment (NAWQA) pilot program in 1986. The long-term goals of the program are to describe the status of and trends in the quality of the Nation's surface- and ground-water resources and to provide a sound, scientific understanding of the primary natural and human factors that affect the quality of these resources.
The Delmarva Peninsula is one of seven pilot project areas in the NAWQA program. As a first step, USGS hydrologists gathered existing water-quality data, interpreted the compiled data, and then developed a preliminary regional waterquality assessment of ground water in the peninsula. Their assessment showed that the existing data were unsuitable for regional analysis because of inconsistent sampling methods, incomplete information on the locations and depth of wells, and the hydrogeology of the aquifers into which the wells tap. The assessment also showed that apparent relations between water-quality constituents, such as nitrate, and ancillary features, such as land use, may depend on the scale of the maps depicting the ancillary data. The implications of these findings are well illustrated by the relation between land
Comparison of mean concentrations of total nitrate plus nitrite, in milligrams per liter, in samples of surface-water runoff, of water in the unsaturated zone, and of ground water from the two field sites having best landmanagement and standard landmanagement practices.
MILLIGRAMS PER LITER
6 FT 3 FT
use and nitrate concentrations in shallow ground water in the peninsula.
The Delmarva Peninsula, located in the Coastal Plain physiographic province, is comprised of most of Delaware and the entire Eastern Shore of Maryland and Virginia. The land area is about 6,050 square miles, of which about 48 percent is used for agriculture. Woodlands constitute about 31 percent of the land area and commonly are interspersed with agricultural areas. The degree to which agricultural lands and woodlands are interspersed depends in large measure on the physiography of the area. Poorly drained uplands, primarily located in the central part of the peninsula, are dominated by relatively small plots with interspersed woodlands and agricultural areas. Well-drained areas, which flank the central upland, typically are dominated by large agricultural tracts. Urban or built-up land, most extensive in the northern part of the peninsula, composes about 7 percent of the land area; the remaining land areas are wetlands or barren.
One of the chief water-quality concerns on the Delmarva Peninsula is elevated nitrate concentrations in shallow ground water, which probably are derived from human-related sources such as septic tank effluents, livestock wastes, and fertilizers. Results from previous studies show that the highest nitrate concentrations are found in agricultural areas and in urban or built-up land having well-drained sediments. Lowest nitrate concentrations are found in woodlands having poorly drained and relatively impermeable sediments.
Nitrate concentrations ranged from 0.1 to 59.0 milligrams per liter (mg/L) as nitrogen in samples collected from 399 wells in the surficial aquifer by USGS personnel from 1944 through 1987, prior to the start of the NAWQA program. The drinking-water standard for nitrate, established by the U.S. Environmental Protection Agency, is 10 mg/L as nitrogen. The initial analysis of the relations between nitrate concentrations in ground water and land use revealed median nitrate concentrations in groundwater samples from the 399 wells of 0.9 mg/L in woodlands, 2.9 mg/L in urban or built-up land, and 4.9 mg/L in agricultural areas.
The differences in these median nitrate concentrations indicate that land use does influence nitrate concentrations in ground water in the Delmarva Peninsula. The observed relations with land use, however, may be affected by the large differences in the depths of the wells (from 3 to about 300 feet) and differences in the time of sampling (from 1944 through 1987). For example, the depth at which the sample was obtained may introduce inaccuracy because deeper ground water may originate from more distant recharge areas that have different land use. The year the sample was obtained may also affect the land-use correlation because of improved methods of preserving samples. Preserving nitrate
ated size for agricultural and woodland areas is 40 acres, and a well located in the middle of a 35-acre farm surrounded by a forest often would be categorized as woodland) and the limited geographic distribution of the 62 wells (more than 35 percent are located in the poorly drained uplands where woodland and agricultural areas are well interspersed).
By using larger scale, more detailed maps (1:24,000 scale) that are shaded to distinguish woodlands from agricultural and urbanized areas, USGS hydrologists were better able to determine the relation of land use to the water-quality data for the 62 wells. Based on this analysis, median nitrate concentrations are 0.6 mg/L in woodlands and 6.7 mg/L in agricultural areas. Median nitrate concentration in urban or built-up land was not assessed because only 2 of the 62 wells are located in urbanized areas. These results are more consistent with the expectation that nitrate concentrations in ground water are lower in woodlands, which suggests that verifying apparent relations between nitrate concentrations and land use may depend upon the availability and use of more detailed land-use data.
The next step was for the hydrologists to collect samples for nitrate analyses from shallow wells that were installed in the surficial aquifer throughout the Delmarva Peninsula as part of the NAWQA project. These newly collected samples also were examined for land-use relations. The wells are located in subregions that have different hydrogeologic and physiographic features. Wells were drilled in woodland and agricultural areas and also in selected sites downgradient from these two predominant land uses in order to assess the impact of movement of nitrate in ground water from one area to another. Locations of the wells were selected without bias toward known or suspected problem areas, and all wells were constructed in the same way. The samples are being collected with consistent sampling methods. The scientists are also keeping a careful record of the predominant land use around (within a quarter-mile radius) and upgradient from the wells. Thirtytwo wells, generally less than 30 feet in depth, were sampled from June through August 1988. Analysis of these samples
concentrations in water samples collected prior to the mid-1970's only included chilling to 39 °F at the time of sampling; the more recent standard procedure includes chilling and adding mercuric chloride, which improves the stability of nitrate concentrations. Therefore, early nitrate data may be misleadingly low.
Problems related to differences in the depth of wells and year of sample collection can be reduced by restricting the analysis to samples collected from shallow wells in the surficial aquifer during recent years. Thus, data since 1980 for nitrate concentrations at 62 of the 399 wells, all having depths less than or equal to 30 feet, were assessed separately. This analysis indicated median nitrate concentrations of 0.9 mg/L in urban or built-up land, 5.8 mg/L in agricultural areas, and 7.0 mg/L in woodlands, surprisingly high for ground water underlying woodlands. This may be caused by the relatively coarse resolution of the land-use data (at the 1:250,000 scale used in this analysis, the minimum differenti
indicate median nitrate concentrations of 0.1 mg/L in woodlands and 4.2 mg/L in agricultural areas. Again, median nitrate concentration in urban or built-up land was not assessed because only 2 of the 32 wells are in urbanized areas. Median nitrate concentrations are 0.1 mg/L in wells downgradient from woodlands and 3.7 mg/L in wells downgradient from agricultural areas. Again, these results are consistent with the expectation that nitrate concentrations in ground water are lower in woodlands.
Additional broad-scale and targeted samples from shallow ground water are being collected as part of the NAWQA project at more than 100 sites distributed among major physiographic, hydrogeologic, and land-use settings in the peninsula. By broadening the sampling sites, the USGS will gain further insight into methods for investigating relations between nitrate and other water-quality constituents and land use.
but that can impair the usefulness of the water for certain purposes.
Troublesome contamination of ground water falls into two basic categories related to the source or sources of the contamination. Point sources of pollution-such as wastedisposal sites, storage-tank leaks, and hazardous chemical spills-can cause high concentrations of a variety of toxic metals, organic chemicals, and petroleum products in ground water in localized areas. These types of local problems commonly occur in densely populated urban and industrialized areas. Contamination can also occur from nonpoint sources, such as agricultural activities or high-density domestic waste disposal (septic systems) in urban centers. Such contamination of ground water on a broad scale can enter shallow wells over large areas and can affect several counties.
A hydraulic connection also exists between surface- and ground-water resources. About 40 percent of the average annual streamflow in the United States is supplied by ground water, and during long dry periods, ground-water discharge provides nearly all of the base flow to streams. Therefore, if a persistent pollutant gets into an aquifer, that pollutant eventually could discharge into a stream.
At present, only a very small percentage of the total volume of potable ground water in the United States is contaminated from both point and nonpoint sources. USGS scientists caution, however, that available data about the occurrence of synthetic organic and toxic substances generally are inadequate to determine the full extent of ground-water contamination in the Nation's aquifers or to define trends in ground-water quality. To date, most information about the occurrence of these substances has come from the study
Ground-Water Quality in the United States - An Overview
Percentage of ground-water use in the United States by major category (Alaska and Hawaii included in percentages).
Ground water is one of the most important natural resources of the United States. Degradation of ground-water quality is an issue of national concern because ground water is a major source for public water supply, for rural domestic use, for irrigation, and for self-supplied industrial use.
Information compiled by the USGS clearly shows that the United States has large amounts of potable ground water available for use. Even though the quality of most of the Nation's ground water is good, ground water in some locations contains one or more naturally occurring chemical constituents or properties that exceed Federal or State drinkingwater standards or otherwise impair use, which include:
Organisms, such as bacteria;
of individual sites or areas where contamination has already been detected or suspected. Contamination in some areas may be more widespread than currently thought and there may be areas where contamination is present, but no studies have yet been conducted.
Furthermore, any assessment of the Nation's ground-water quality must be tempered by the fact that most water-quality descriptions largely are based on analyses of inorganic chemicals. Ground-water analyses of toxic constituents and synthetic organic chemicals are relatively scarce, but available data have led to the growing realization that these chemicals locally have contaminated shallow aquifers in many parts of the country. Also, existing data generally have been collected to monitor the quality of drinking-water supplies or to evaluate waste sites but not to provide
an assessment of quality of ground-water resources where they occur.
Reports of contamination are likely to increase in the coming years as the search for contamination intensifies and as more sophisticated analytical techniques are used to detect trace amounts of toxic constituents and synthetic organic chemicals in water. The presence of organic or toxic chemicals in very small concentrations in ground water does not necessarily imply a health or environmental threat, but their presence does raise questions about the source of the chemicals and the possibility that concentrations might increase to toxic levels over time. It is on these issues that the USGS is concentrating much of its research in ground-water contamination.