SHALLOW GROUNDWATER QUALITY BENEATH A MAJOR URBAN CENTER - DENVER, COLORADO, USA

A survey of the chemical quality of ground water in the unconsolidated alluvial aquifer beneath a major urban center (Denver, Colorado, USA) was performed in 1993 with the objective of characterizing the qualit y of shallow ground-water in the urban area and relating water quality to land use. Thirty randomly selected alluvial wells were each sample d once for a broad range of dissolved constituents. The urban land use at each well site was sub-classified into one of three land-use setti ngs: residential, commercial, and industrial. Shallow ground-water qua lity was highly variable in the urban area and the variability could b e related to these land-use setting classifications. Sulfate (SO4) was the predominant anion in most samples from the residential and commer cial land-use settings, whereas bicarbonate (HCO3) was the predominant anion in samples from the industrial land-use setting, indicating a p ossible shift in redox conditions associated with land use. Only three of 30 samples had nitrate concentrations that exceeded the US nationa l drinking-water standard of 10 mg l(-1) as nitrogen, indicating that nitrate contamination of shallow ground water may not be a serious pro blem in this urban area. However, the highest median nitrate concentra tion (4.2 mg l(-1)) was in samples from the residential setting, where fertilizer application is assumed to be most intense. Twenty-seven of 30 samples had detectable pesticides and nine of 82 analyzed pesticid e compounds were detected at low concentrations, indicating that pesti cides are widely distributed in shallow ground water in this urban are a. Although the highest median total pesticide concentration (0.17 mu g l(-1)) was in the commercial setting, the herbicides prometon and at razine were found in each land-use setting. Similarly, 25 of 29 sample s analyzed had detectable volatile organic compounds (VOCs) indicating these compounds are also widely distributed in this urban area. The t otal VOC concentrations in sampled wells ranged from nondetectable to 23 442 mu g l(-1). Widespread detections and occasionally high concent rations point to VOCs as the major anthropogenic ground-water impact i n this urban environment. Generally, the highest VOC concentrations oc curred in samples from the industrial setting. The most frequently det ected VOC was the gasoline additive methyl tert-butyl ether (MTBE, in 23 of 29 wells). Results from this study indicate that the quality of shallow ground water in major urban areas can be related to land-use s ettings. Moreover, some VOCs and pesticides may be widely distributed at low concentrations in shallow ground water throughout major urban a reas. As a result, the differentiation between point and non-point sou rces for these compounds in urban areas may be difficult.