WATER QUALITY IN SHALLOW ALLUVIAL AQUIFERS,UPPER COLORADO RIVER BASIN,COLORADO, 19971

ABSTRACT: Shallow ground water in areas of increasing urban development within the Upper Colorado River Basin was sampled for inorganic and organic constituents to characterize water‐quality conditions and to identify potential anthropogenic effects resulting from development. In 1997, 25 shallow monitoring wells were installed and sampled in five areas of urban development in Eagle, Grand, Gunnison, and Summit Counties, Colorado. The results of this study indicate that the shallow ground water in the study area is suitable for most uses. Nonparametric statistical methods showed that constituents and parameters measured in the shallow wells were often significantly different between the five developing urban areas. Radon concentrations exceeded the proposed USEPA maximum contaminant level at all sites. The presence of nutrients, pesticides, and volatile organic compounds indicate anthropogenic activities are affecting the shallow ground‐water quality in the study area. Nitrate as N concentrations greater than 2.0 mg/L were observed in ground water recharged between the 1980s and 1990s. Low concentrations of methylene blue active substances were detected at a few sites. Total coliform bacteria were detected at ten sites; however, E. coli was not detected. Continued monitoring is needed to assess the effects of increasing urban development on the shallow ground‐water quality in the study area.     

CONTRASTING WATER QUALITY FROM PAIRED DOMESTIC/PUBLIC SUPPLY WELLS,CENTRAL HIGH PLAINS1

ABSTRACT: Closely located domestic and public supply wells were sampled using identical sampling procedures to allow comparison of water quality associated with well type. Water samples from 15 pairs of wells with similar screened intervals completed in the central High Plains regional aquifer in parts of Kansas, Oklahoma, and Texas were analyzed for more than 200 water quality constituents. No statistically significant differences were observed between the concentrations of naturally‐derived constituents (major ions, trace elements, and radon) in paired wells. However, differences in water quality between paired wells were observed for selected anthropogenic compounds (pesticides and tritium), in that some public supply wells produced water that was more recently recharged and contained constituents derived from surface activities. The presence of recently recharged water and compounds indicative of anthropogenic activities in some public supply wells was likely due to operational variations (pumping rate and pumping cycles), as demonstrated in a particle tracking simulation. Water containing surface‐derived anthropogenic compounds from near the water table was more quickly drawn to high volume public supply wells (less than five years) than domestic wells (greater than 120 years) with small pumping rates. These findings indicate that water quality samples collected from different well types in the same area are not necessarily directly comparable. Sampling domestic wells provides the best broad‐scale assessment of water quality in this aquifer setting because they are less susceptible to localized contamination from near the water table. However, sampling public supply wells better represents the quality of the used resource because of the population served.     

VOLATILE ORGANIC COMPOUNDS IN GROUND WATER FROM RURAL PRIVATE WELLS, 1986 TO 19991

ABSTRACT: The U.S. Geological Survey (USGS) collected or compiled data on volatile organic compounds (VOCs) in samples of untreated ground water from 1,926 rural private wells during 1986 to 1999. At least one VOC was detected in 12 percent of samples from rural private wells. Individual VOCs were not commonly detected with the seven most frequently detected compounds found in only 1 to 5 percent of samples at or above a concentration of 0.2 microgram per liter (μg/l). An assessment level of 0.2 μg/l was selected so that comparisons of detection frequencies between VOCs could be made. The seven most frequently detected VOCs were: trichloromethane, methyl tert‐butyl ether, tetrachloroethene, dichlorodifluoromethane, methylbenzene, 1,1,1‐trichloroethane, and 1,2‐dibromo‐3‐chloropropane. Solvents and trihalomethanes were the most frequently detected VOC groups in private wells. The distributions of detections of gasoline oxygenates and fumigants seemed to be related to the use patterns of compounds in these groups. Mixtures were a common mode of occurrence of VOCs with one‐quarter of all samples with detections including two or more VOCs. The concentrations of most detected VOCs were relatively small and only 1.4 percent of samples had one or more VOC concentrations that exceeded a federally established drinking water standard or health criterion.     

GEOCHEMICAL CHARACTERIZATION OF SHALLOW GROUND WATER IN THE EUTAW AQUIFER,MONTGOMERY, ALABAMA1

ABSTRACT: Ground water samples were collected from 30 wells located in, or directly down gradient from, recharge areas of the Eutaw aquifer in Montgomery, Alabama. The major ion content of the water evolves from calcium‐sodium‐chloride‐dominated type in the recharge area to calcium‐bicarbonate‐dominated type in the confined portion of the aquifer. Ground water in the recharge area was under saturated with respect to aluminosilicate and carbonate minerals. Ground water in the confined portion of the aquifer was at equilibrium levels for calcite and potassium feldspar. Dissolved oxygen and nitrite‐plus‐nitrate concentrations decreased as ground water age increased; pH, iron, and sulfate concentrations increased as ground water age increased. Aluminum, copper, and zinc concentrations decreased as ground water age and pH increased. These relations indicate that nitrate, aluminum, copper, and zinc are removed from solution as water moves from recharge areas to the confined areas of the Eutaw aquifer. The natural evolution of ground water quality, which typically increases the pH and decreases the dissolved oxygen content, may be an important limiting factor to the migration of nitrogen based compounds and metals.     

GROUND AND SURFACE WATER QUALITY IMPACTS OF NORTH CAROLINA SANITARY LANDFILLS1

ABSTRACT: Ground and surface water quality monitoring data from 71 municipal sanitary landfills in North Carolina were analyzed to determine the nature and extent of current contamination problems and identify any common characteristics associated with this contamination. A total of 322 surface and 411 ground water quality records were analyzed using the SAS data system. Almost all the landfill records included inorganic and heavy metal analyses while approximately half of the records also included organic analyses by CC/MS. Our analysis indicates that landfills are having measurable impacts on ground and surface water quality, but these impacts may not be as severe as is commonly assumed. Statistically significant increases were detected in the average concentrations in ground water and downstream surface water samples when compared to upstream surface water samples. The largest percentage increases were observed for zinc, turbidity, total organic carbon, conductivity, total dissolved solids, and lead. Violations of ground water quality standards for heavy metals and hazardous organic compounds were detected at 53 percent of the landfills where adequate data existed. The moat common heavy metal violations were for lead (18 percent), chromium (18 percent), zinc (6 percent), cadmium (6 percent), and arsenic (6 percent) (percentage of landfills violating shown in parenthesis). The organic compounds that appear to pose the greatest threat to ground water are the chlorinated solvents (8 percent), petroleum derived hydrocarbons (8 percent), and pesticides (5 percent). A comparison of monitoring data from sanitary landfills and secondary wastewater treatment plants suggests that the concentrations of heavy metal and organic pollutants discharged to surface waters from these two sources are similar.     

NITRATE CONTAMINATION IN OREGON WELL WATER: GEOLOGIC VARIABILITY AND THE PUBLIC'S PERCEPTION1

ABSTRACT: Approximately 4.5 million people in the United States who rely on well water are exposed to nitrate‐N concentrations exceeding the 10 mg/l standard. In this study in the Southern Willamette Valley in Oregon we reassessed nitrate‐N in rural wells sampled in 2000–2001, compared nitrate‐N concentrations among geological units, and surveyed residents about their perceptions of well water quality. Nitrate‐N concentrations were again sampled in 2002 and found to have increased significantly from the previous period. With rapid population growth in the area, the potential health risk in drinking well water that exceeds 10 mg/l nitrate‐N warrants continued public education. Nitrate‐N concentrations were found to be higher in the Holocene alluvium of the Willamette River and the Pleistocene sand and gravel post‐Missoula Flood deposits. Researchers conducting future studies may choose to stratify and monitor wells by geologic unit and by other parameters that estimate input of nutrients to the environment. Opinions differed between agricultural landowners and nonagricultural landowners with regard to the impact that agricultural fertilizers may have on water quality. Participants were supportive of a range of regulatory actions that might be used by homeowners or landowners to address ground water contamination. Given that the area is now designated a Groundwater Management Area, understanding local stakeholders’perceptions is critical and strategic and has the potential to help public agencies manage potential conflicts of opinion among stakeholders, build consensus, and help guide the approach to restoring ground water quality.     

HERBICIDES AND DEGRADATES IN SHALLOW AQUIFERS OF ILLINOIS: SPATIAL AND TEMPORAL TRENDS1

ABSTRACT: During the fall of 2000, the occurrence was examined of 16 herbicides and 13 herbicide degradates in samples from 55 wells in shallow aquifers underlying grain producing regions of Illinois. Herbicide compounds with concentrations above 0.05 μg/L were detected in 56 percent of the samples. No concentrations exceeded regulatory drinking water standards. The six most frequently detected compounds were degradates. Water age was an important factor in determining vulnerability of ground water to transport of herbicide compounds. Unconsolidated aquifers, which were indicated to generally contain younger ground water than bedrock aquifers, had a higher occurrence of herbicides (73 percent of samples) than bedrock aquifers (22 percent). Temporal analysis to determine if changes in concentrations of selected herbicides and degradates could be observed over a near decadal period indicated a decrease in detection frequency (25 to 18 percent) between samplings in 1991 and 2000. Over this period, significant differences in concentrations were observed for atrazine (decrease) and total acetochlor (increase). The increase in acetochlor compound concentrations corresponds to an increase in acetochlor use during the study period, while the decrease in atrazine concentrations corresponds to relatively consistent use of atrazine. Changes in frequency of herbicide detection and concentration do not appear related to changes in land use near sampled wells.     

AQUIFER SALINIZATION IN THE YUN‐LIN COASTAL AREA,TAIWAN1

ABSTRACT: This study analyzes possible causes of shallow ground water salinization in the coastal area of Yun‐Lin. The local hydro‐geologic setting is determined from geological drilling data and sea floor topography. Three possible causes (sea water intrusion, salt water percolation through wells, and infiltration of salty water from fish ponds) are evaluated. Chloride concentration is used as an index to measure ground water salinization. Sea water intrusion is modeled by the advective/dispersive equation, and salt water infiltration from wells and fish ponds is calculated by estimating the amount of water percolated. The determined local hydrogeologic setting suggests that the shallow aquifer may be connected to the sea water, resulting in salt water intrusion as a large amount of shallow ground water is withdrawn. The percent contributions of sea water intrusion, percolation through wells, and infiltration of water from fish ponds, to the salinization of the shallow aquifer at Ko‐Hu in the Yun‐Lin coastal area are approximately 27 percent, less than 1 percent and 73 percent, respectively. The results suggest that the vertical infiltration of salt water from fish ponds is the major cause of shallow ground water salinization in the coastal area of Yun‐Lin.     

ANOMALOUS NATURAL SULFATE IN WATER WELLS OF THE GREENBRIER GROUP,WEST VIRGINIA1

ABSTRACT: Many water wells developed in the Middle Mississippian Greenbrier Group of central Greenbrier County, West Virginia, are very productive because of the abundant solution conduits in this karst aquifer. Water from these wells, all of which (with one exception) are clustered in a small area about eight kilometers northwest of Lewisburg, West Virginia, is typically very hard (calcium-magnesium-bicarbonate type). Of 74 wells sampled, eight showed sulfate concentrations ranging from 600 to 1700 mg/l. These wells also showed a much higher than average concentration of calcium, magnesium, chloride, sodium, and, in a few cases, iron. The water from several of these wells was not potable. Previously unpublished work has suggested that dolomite and gypsum dissolution have contributed magnesium and sulfate ions to the ground water. It is also likely that, where sodium, chloride, and iron concentrations are above background, halite dissolution and pyrite oxidation are contributing to the problem. All of these minerals are often associated with the basal waterbearing strata of the Greenbrier Group. In addition, it is possible that the wells are receiving deeply circulating ground waters, via fracture zones, which feed many other well-known sulfur and/or thermal springs in the central Appalachians.     

INCREASING WATER SUPPLY BY MIXING OF FRESH AND SALINE GROUND WATERS1

ABSTRACT: The quality of ground water in any aquifer takes its final form due to natural mixture of waters, which may originate from different sources. Water quality varies from one aquifer to another and even within the same aquifer itself. Different ground water quality is obtained from wells and is mixed in a common reservoir prior to any consumption. This artificial mixing enables an increase in available ground water of a desired quality for agricultural or residential purposes. The question remains as to what proportions of water from different wells should be mixed together to achieve a desired water quality for this artificial mixture. Two sets of laboratory experiments were carried out, namely, the addition of saline water to a fixed volume of fresh water. After each addition, the mixture volume and the electric conductivity value of the artificially mixed water were recorded. The experiments were carried out under the same laboratory temperature of 20°C. A standard curve was developed first experimentally and then confirmed theoretically. This curve is useful in determining either the volume or discharge ratio from two wells to achieve a predetermined electrical conductivity value of the artificial mixture. The application of the curve is given for two wells within the Quaternary deposits in the western part of the Kingdom of Saudi Arabia.     

WALNUT CREEK WATERSHED MONITORING PROJECT,IOWA MONITORING WATER QUALITY IN RESPONSE TO PRAIRIE RESTORATION1

ABSTRACT: Land use and surface water data for nitrogen and pesticides (1995 to 1997) are reported for the Walnut Creek Watershed Monitoring Project, Jasper County Iowa. The Walnut Creek project was established in 1995 as a nonpoint source monitoring program in relation to watershed habitat restoration and agricultural management changes implemented at the Neal Smith National Wildlife Refuge by the U.S. Fish and Wildlife Service. The monitoring project utilizes a paired‐watershed approach (Walnut and Squaw creeks) as well as upstream/downstream comparisons on Walnut for analysis and tracking of trends. From 1992 to 1997, 13.4 percent of the watershed was converted from row crop to native prairie in the Walnut Creek watershed. Including another 6 percent of watershed farmed on a cash‐rent basis, land use changes have been implemented on 19.4 percent of the watershed by the USFWS. Nitrogen and pesticide applications were reduced an estimated 18 percent and 28 percent in the watershed from land use changes. Atrazine was detected most often in surface water with frequencies of detection ranging from 76–86 percent. No significant differences were noted in atrazine concentrations between Walnut and Squaw Creek. Nitrate‐N concentrations measured in both watersheds were similar; both basins showed a similar pattern of detection and an overall reduction in nitrate‐N concentrations from upstream to downstream monitoring sites. Water quality improvements are suggested by nitrate‐N and chloride ratios less than one in the Walnut Creek watershed and low nitrate‐N concentrations measured in the subbasin of Walnut Creek containing the greatest amount of land use changes. Atrazine and nitrate‐N concentrations from the lower portion of the Walnut Creek watershed (including the prairie restoration area) may be decreasing in relation to the upstream untreated component of the watershed. The frequencies of pesticide detections and mean nitrate‐N concentrations appear related to the percentage of row crop in the basins and subbasins. Although some results are encouraging, definitive water quality improvements have not been observed during the first three years of monitoring. Possible reasons include: (1) more time is needed to adequately detect changes; (2) the size of the watershed is too large to detect improvements; (3) land use changes are not located in the area of the watershed where they would have greatest effect; or (4) water quality improvements have occurred but have been missed by the project monitoring design. Longer‐term monitoring will allow better evaluation of the impact of restoration activities on water quality.     

ARSENIC CONSUMPTION AND HEALTH RISK PERCEPTIONS IN A RURAL WESTERN U.S. AREA1

ABSTRACT: Churchill County, Nevada, has approximately 23,000 residents, among whom an estimated 13,500 relied on private wells for water supply in 2002. This study examined exposure to arsenic in water supplies among residents with private domestic wells and factors related to householder choice to consume tap water. It compared opinions and concerns about water quality with consumption habits and observed concentrations from tap water samples. The results from 351 households indicated that a majority (75 percent) of respondents consumed tap water and that a minority (38 percent) applied treatment. Approximately 66 percent of those who consumed tap water were exposed to concentrations of arsenic that exceeded 10 ppb. Water consumption was related to application of treatment. Among 98 respondents who were not at all concerned about the health effects of aqueous arsenic, 59 (60 percent) reported consuming tap water with concentrations of arsenic exceeding 10 ppb. Conversely, among 86 respondents who were highly concerned about arsenic, 33 (37 percent) consumed tap water with concentrations of arsenic exceeding 10 ppb. Results from a national sampling effort showed that 620 of 5,304 private wells sampled (11.7 percent) had arsenic concentrations above 10 ppb. The paradox of awareness of arsenic in water supplies coupled with consumption of aqueous arsenic in concentrations greater than 10 ppb may be common in other parts of the nation. Enhanced educational efforts, especially related to tap water sampling and explanations of efficacy of available treatment, may be useful means of reducing exposure through private water supplies.     

HERBICIDES AND TRANSFORMATION PRODUCTS IN SURFACE WATERS OF THE MIDWESTERN UNITED STATES1

ABSTRACT: Most herbicides applied to crops are adsorbed by plants or transformed (degraded) in the soil, but small fractions are lost from fields and either move to streams in overland runoff, near surface flow, or subsurface drains, or they infiltrate slowly to ground water. Herbicide transformation products (TPs) can be more or less mobile and more or less toxic in the environment than their source herbicides. To obtain information on the concentrations of selected herbicides and TPs in surface waters of the Midwestern United States, 151 water samples were collected from 71 streams and five reservoir outflows in 1998. These samples were analyzed for 13 herbicides and 10 herbicide TPs. Herbicide TPs were found to occur as frequently or more frequently than source herbicides and at concentrations that were often larger than their source herbicides. Most samples contained a mixture of more than 10 different herbicides or TPs. The ratios of TPs to herbicide concentrations can be used to determine the source of herbicides in streams. Results of a two‐component mixing model suggest that on average 90 percent or more of the herbicide mass in Midwestern streams during early summer runoff events originates from the runoff and 10 percent or less comes from increased ground water discharge.     

GROUND WATER NITRATE REDUCTION IN GIANT CANE AND FOREST RIPARIAN BUFFER ZONES1

ABSTRACT: Ground water contamination by excess nitrate leaching in row‐crop fields is an important issue in intensive agricultural areas of the United States and abroad. Giant cane and forest riparian buffer zones were monitored to determine each cover type's ability to reduce ground water nitrate concentrations. Ground water was sampled at varying distances from the field edge to determine an effective width for maximum nitrate attenuation. Ground water samples were analyzed for nitrate concentrations as well as chloride concentrations, which were used as a conservative ion to assess dilution or concentration effects within the riparian zone. Significant nitrate reductions occurred in both the cane and the forest riparian buffer zones within the first 3.3 m, a relatively narrow width. In this first 3.3 m, the cane and forest buffer reduced ground water nitrate levels by 90 percent and 61 percent, respectively. Approximately 40 percent of the observed 99 percent nitrate reduction over the 10 m cane buffer could be attributed to dilution by upwelling ground water. Neither ground water dilution nor concentration was observed in the forest buffer. The ground water nitrate attenuation capabilities of the cane and forest riparian zones were not statistically different. During the spring, both plant assimilation and denitrification were probably important nitrate loss mechanisms, while in the summer nitrate was more likely lost via denitrification since the water table dropped below the rooting zone.     

Pesticide and Transformation Product Detections and Age‐Dating Relations from Till and Sand Deposits1

Abstract:  Pesticide and transformation product concentrations and frequencies in ground water from areas of similar crop and pesticide applications may vary substantially with differing lithologies. Pesticide analysis data for atrazine, metolachlor, alachlor, acetochlor, and cyanazine and their pesticide transformation products were collected at 69 monitoring wells in Illinois and northern Indiana to document occurrence of pesticides and their transformation products in two agricultural areas of differing lithologies, till, and sand. The till is primarily tile drained and has preferential fractured flow, whereas the sand primarily has surface water drainage and primary porosity flow. Transformation products represent most of the agricultural pesticides in ground water regardless of aquifer material – till or sand. Transformation products were detected more frequently than parent pesticides in both the till and sand, with metolachlor ethane sulfonic acid being most frequently detected. Estimated ground‐water recharge dates for the sand were based on chlorofluorocarbon analyses. These age‐dating data indicate that ground water recharged prior to 1990 is more likely to have a detection of a pesticide or pesticide transformation product. Detections were twice as frequent in ground water recharged prior to 1990 (82%) than in ground water recharged on or after 1990 (33%). The highest concentrations of atrazine, alachlor, metolachlor, and their transformation products, also were detected in samples from ground water recharged prior to 1990. These age/pesticide detection relations are opposite of what would normally be expected, and may be the result of preferential flow and/or ground‐water mixing between aquifers and aquitards as evident by the detection of acetochlor transformation products in samples with estimated ground‐water ages predating initial pesticide application.     

Assessment of contamination from arsenical pesticide use on orchards in the Great Valley region, Virginia and West Virginia, USA

Lead arsenate pesticides were widely used in apple orchards from 1925 to 1955. Soils from historic orchards in four counties in Virginia and West Virginia contained elevated concentrations of As and Pb, consistent with an arsenical pesticide source. Arsenic concentrations in approximately 50% of the orchard site soils and approximately 1% of reference site soils exceed the USEPA Preliminary Remediation Goal (PRG) screening guideline of 22 mg kg(-1) for As in residential soil, defined on the basis of combined chronic exposure risk. Approximately 5% of orchard site soils exceed the USEPA PRG for Pb of 400 mg kg(-1) in residential soil; no reference site soils sampled exceed this value. A variety of statistical methods were used to characterize the occurrence, distribution, and dispersion of arsenical pesticide residues in soils, stream sediments, and ground waters relative to landscape features and likely background conditions. Concentrations of Zn, Pb, and Cu were most strongly associated with high developed land density and population density, whereas elevated concentrations of As were weakly correlated with high orchard density, consistent with a pesticide residue source. Arsenic concentrations in ground water wells in the region are generally <0.005 mg L(-1). There was no spatial association between As concentrations in ground water and proximity to orchards. Arsenic had limited mobility into ground water from surface soils contaminated with arsenical pesticide residues at concentrations typically found in orchards.     

Land management impacts on dairy-derived dissolved organic carbon in ground water

Dairy operations have the potential to elevate dissolved organic carbon (DOC) levels in ground water, where it may interact with organic and inorganic contaminants, fuel denitrification, and may present problems for drinking water treatment. Total and percent bioavailable DOC and total and carbon-specific trihalomethane (THM) formation potential (TTHMFP and STHMFP, respectively) were determined for shallow ground water samples from beneath a dairy farm in the San Joaquin Valley, California. Sixteen wells influenced by specific land management areas were sampled over 3 yr. Measured DOC concentrations were significantly elevated over the background as measured at an upgradient monitoring well, ranging from 13 to 55 mg L(-1) in wells downgradient from wastewater ponds, 8 to 30 mg L(-1) in corral wells, 5 to 12 mg L(-1) in tile drains, and 4 to 15 mg L(-1) in wells associated with manured fields. These DOC concentrations were at the upper range or greatly exceeded concentrations in most surface water bodies used as drinking water sources in California. DOC concentrations in individual wells varied by up to a factor of two over the duration of this study, indicating a dynamic system of sources and degradation. DOC bioavailability over 21 d ranged from 3 to 10%, comparable to surface water systems and demonstrating the potential for dairy-derived DOC to influence dissolved oxygen concentrations (nearly all wells were hypoxic to anoxic) and denitrification. TTHMFP measurements across all management units ranged from 141 to 1731 microg L(-1), well in excess of the maximum contaminant level of 80 microg L(-1) established by the Environmental Protection Agency. STHMFP measurements demonstrated over twofold variation ( approximately 4 to approximately 8 mmol total THM/mol DOC) across the management areas, indicating the dependence of reactivity on DOC composition. The results indicate that land management strongly controls the quantity and quality of DOC to reach shallow ground water and hence should be considered when managing ground water resources and in any efforts to mitigate contamination of ground water with carbon-based contaminants, such as pesticides and pharmaceuticals.     

222RN IN THE WATER DISTRIBUTION SYSTEM OF TUCSON,ARIZONA1

ABSTRACT: A study of ²²²Rn concentrations in the water distribution system of Tucson, Arizona, revealed levels of 60 to 1260 pCi/L in domestic waters. These measurements are comparable to levels of between 80 and 1400 pCi/l for ²²²Rn found in ground water samples in the North-Central Tucson basin (Kahn et al., 1994). Estimated loss of ²²²Rn due to radioactive decay during travel from the well head to the home ranges from 8 to 50 percent.     

EFFECTS OF IRRIGATION ON WATER QUALITY OF A SHALLOW UNCONFINED AQUIFER1

ABSTRACT: The ground water quality of a shallow unconfined aquifer was monitored before and after implementation of a border strip irrigation scheme, by taking monthly samples from an array of 13 shallow wells. Two 30 m deep wells were sampled to obtain vertical concentration profiles. Marked vertical, temporal, and spatial variabilities were recorded. The monthly data were analyzed for step and linear trends using nonparametric tests that were adjusted for the effects of serial correlation. Average nitrate concentrations increased in the preirrigation period and decreased after irrigation began. This was attributed to wetter years in 1978–1979 than in 1976–1977 which increased leaching, and to disturbance of the topsoil during land contouring before irrigation, followed by excessive drainage after irrigation. Few significant trends were recorded for other determinants, possibly because of shorter data records. Nitrate, sulphate, and potassium concentrations decreased with depth, whereas sodium, calcium, bicarbonate, and chloride concentrations increased. These trends allowed an estimation to be made of the depth of ground water affected by percolating drainage. This depth increased during the irrigation season and after periods of winter recharge. Furthermore, an overall increase in the depth of drainage-affected ground water occurred with time, which paralleled the development of the irrigation scheme.     

Transport and fate of nitrate and pesticides: hydrogeology and riparian zone processes

There is continuing concern over potential impacts of widespread application of nutrients and pesticides on ground- and surface-water quality. Transport and fate of nitrate and pesticides were investigated in a shallow aquifer and adjacent stream, Cow Castle Creek, in Orangeburg County, South Carolina. Pesticide and pesticide degradate concentrations were detected in ground water with greatest frequency and largest concentrations directly beneath and downgradient from the corn (Zea mays L.) field where they were applied. In almost all samples in which they were detected, concentrations of pesticide degradates greatly exceeded those of parent compounds, and were still present in ground waters that were recharged during the previous 18 yr. The absence of both parent and degradate compounds in samples collected from deeper in the aquifer suggests that this persistence is limited or that the ground water had recharged before use of the pesticide. Concentrations of NO(-)(3) in ground water decreased with increasing depth and age, but denitrification was not a dominant controlling factor. Hydrologic and chemical data indicated that ground water discharges to the creek and chemical exchange takes place within the upper 0.7 m of the streambed. Ground water had its greatest influence on surface-water chemistry during low-flow periods, causing a decrease in concentrations of Cl(-), NO(-)(3), pesticides, and pesticide degradates. Conversely, shallow subsurface drainage dominates stream chemistry during high-flow periods, increasing stream concentrations of Cl(-), NO(-)(3), pesticides, and pesticide degradates. These results point out the importance of understanding the hydrogeologic setting when investigating transport and fate of contaminants in ground water and surface water.