The history and fate of groundwater nitrate (NO•-) contamination were compared in 2 small adjacent agricultural watersheds in the Atlantic coastal plain by combined use of chronologic (CC12F2, 3H), chemical (dissolved solids, gases), and isotopic (•15N, •13C, •34S) analyses of recharging groundwaters, discharging groundwaters, and surface waters. The results demonstrate the interactive effects of changing agricultural practices, groundwater residence times, and local geologic features on the transfer of NO•through local flow systems. Recharge dates of groundwaters taken in 1990-1992 from the surficial aquifer in the Chesterville Branch and Morgan Creek watersheds near Locust Grove, Maryland, ranged from pre-1940 to the late 1980's.When corrected for localized denitrification by use of dissolved gas concentrations, the dated waters provide a 40-year record of the recharge rate of NO•, which increased in both watersheds by a factor of 3-6, most rapidly in the 1970's. The increase in groundwater NO•-over time was approximately proportional to the documented increase in regional N fertilizer use, and could be accounted for by oxidation and leaching of about 20-35% of the fertilizer N. Groundwaters discharging upward beneath streams in both watersheds had measured recharge dates from pre-1940 to 1975, while chemical data for second-order reaches of the streams indicated average groundwater residence times in the order of 20+ years. At the time of the study, NO•-discharge rates were less than NO•-recharge rates for at least two reasons: (1) discharge of relatively old waters with low initial NO•-concentrations, and (2) local denitrification. In the Chesterville Branch watershed, groundwaters remained oxic throughout much of the surficial aquifer and discharged relatively unaltered to the stream, which had a relatively high NO• concentration (9-10 mg/L as N). In the Morgan Creek watershed, groundwaters were largely reduced and denitrified before discharging to the stream, which had a relatively low NO•-concentration (2-3 mg/L as N). Chemical and isotopic data indicate that quantitative denitrification occurred within buried calcareous glauconitic marine sediments that are present at relatively shallow depths beneath the Morgan Creek watershed. NO•-removal by forests, wetlands, and shallow organic-rich soils in near-stream environments was largely avoided by groundwaters that followed relatively deep flow paths before converging and discharging rapidly upward to the streams. This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union.Paper number 95WR01584. and although many potential causes of such imbalances are known, the essential causes in specific field situations are not always easy to resolve. Watershed NO•-gradients can have a variety of different configurations that depend locally on both transient and steady state variables, including (1) changes in the N contents of recharging groundwaters over time (related to changes in land use practices), (2) the distance, direction, a...

/ Maryland, Virginia, and Pennsylvania, USA, have agreed to reduce nutrient loadings to Chesapeake Bay by 40% by the year 2000. This requires control of nonpoint sources of nutrients, much of which comes from agriculture. Riparian forest buffer systems (RFBS) provide effective control of nonpoint source (NPS) pollution in some types of agricultural watersheds. Control of NPS pollution is dependent on the type of pollutant and the hydrologic connection between pollution sources, the RFBS, and the stream. Water quality improvements are most likely in areas of where most of the excess precipitation moves across, in, or near the root zone of the RFBS. In areas such as the Inner Coastal Plain and Piedmont watersheds with thin soils, RFBS should retain 50%-90% of the total loading of nitrate in shallow groundwater, sediment in surface runoff, and total N in both surface runoff and groundwater. Retention of phosphorus is generally much less. In regions with deeper soils and/or greater regional groundwater recharge (such as parts of the Piedmont and the Valley and Ridge), RFBS water quality improvements are probably much less. The expected levels of pollutant control by RFBS are identified for each of nine physiographic provinces of the Chesapeake Bay Watershed. Issues related to of establishment, sustainability, and management are also discussed.Research is sometimes applied to broad-scale environmental issues with inadequate knowledge or incomplete understanding. Public policies to encourage or require landscape management techniques such as riparian (streamside) management will often need to proceed with best professional judgment decisions based on incomplete understanding.Riparian forest buffer systems (RFBS) are streamside ecosystems managed for the enhancement of water quality through control of nonpoint source pollution (NPS) and protection of the stream environment. The use of riparian management zones is relatively well established as a best management practice (BMP) for water quality improvement in forestry practices (Comer-

The ability of natural attenuation to mitigate agricultural nitrate contamination in recharging aquifers was investigated in four important agricultural settings in the United States. The study used laboratory analyses, field measurements, and flow and transport modeling for monitoring well transects (0.5 to 2.5 km in length) in the San Joaquin watershed, California, the Elkhorn watershed, Nebraska, the Yakima watershed, Washington, and the Chester watershed, Maryland. Ground water analyses included major ion chemistry, dissolved gases, nitrogen and oxygen stable isotopes, and estimates of recharge date. Sediment analyses included potential electron donors and stable nitrogen and carbon isotopes. Within each site and among aquifer‐based medians, dissolved oxygen decreases with ground water age, and excess N2 from denitrification increases with age. Stable isotopes and excess N2 imply minimal denitrifying activity at the Maryland and Washington sites, partial denitrification at the California site, and total denitrification across portions of the Nebraska site. At all sites, recharging electron donor concentrations are not sufficient to account for the losses of dissolved oxygen and nitrate, implying that relict, solid phase electron donors drive redox reactions. Zero‐order rates of denitrification range from 0 to 0.14 μmol N L−1d−1, comparable to observations of other studies using the same methods. Many values reported in the literature are, however, orders of magnitude higher, which is attributed to a combination of method limitations and bias for selection of sites with rapid denitrification. In the shallow aquifers below these agricultural fields, denitrification is limited in extent and will require residence times of decades or longer to mitigate modern nitrate contamination.

Concentrations of the chlorofluorocarbons (CFCs) CFC-11 and CFC-12 were determined in groundwater from coastal plain sediments of the Delmarva Peninsula. CFC-modeled ages were calculated independently for CFC-11 and CFC-12, and agreed to within 2-3 years in the majority of the waters. Recharge temperatures, determined from dissolved nitrogen and argon concentrations, varied from 9 -+ 2øC over most of the peninsula to 14 +_ 2øC at the southernmost tip of the peninsula in Virginia. The CFC-modeled ages were examined in relation to the known hydrogeologic environment, both on regional scales and in more intensively sampled local scale networks. The CFC-modeled recharge years and measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritium plus 3He distribution. Most of the present distribution of tritium in Delmarva groundwater is consistent with low dispersivities. The results of the study strongly support the use of CFCs for dating shallow, aerobic groundwater. INTRODUCTION Chlorofluorocarbons (CFCs, Freons, (The use of brand names in this report is for identification purposes only and does not represent endorsement by the U.S. Geological Survey) chlorofluoromethanes, halomethanes, or halocarbons), specifically CFC-11 (CCI3F) and CFC-12 (CC12F2), are chemically stable, manmade compounds that have been manufactured since the !940s and 1930s, respectively, for use as propellants in aerosol cans, as foaming agents in plastics, and as refrigerants and solvents. They have received widespread attention in recent years because of their long atmospheric lifetimes, their contribution to ozone destruction in the lower stratosphere, and their contribution as atmospheric greenhouse gases. Growth curves of atmospheric concentrations of CFC-11 and CFC-12 have been reconstructed using production records and release data prior to 1977 and worldwide atmospheric measurements thereafter [Elkins et al., 1993; Busenberg et al., 1993]. Based on these growth curves and CFC solubility relations [Warner and Weiss, 1985], concentrations of CFCs found in groundwater have been used as indicators of the time since the infiltrating groundwater was recharged and isolated from the soil air assuming that the soil air is in equilibrium with the troposphere [Busenberg and Plumruer, 1992]. Results from previous studies confirm the utility of CFCs as dating tools and groundwater tracers [Randall and Schultz, 1976; Schultz et al.; Plummer et al., 1993a, b]. Busenberg and Plumruer [1992] introduced a sampling procedure for collection and storage of water samples in borosilicate glass ampules that are flame sealed in the field. CFCs remain stable in these ampules for many months and can be returned to the laboratory for analysis. Plumruer et al. [ 1993a] summarize the CFC dating method, evaluate effects of hydrodynamic dispersion on CFC dating, and compare CFC dating to dating with other environmental tracers. The present study uses CFC-11 and CFC-12 to date shallow groundwater from largely u...