Herbicide compounds were prevalent in ground water across Iowa, being detected in 70% of the 106 municipal wells sampled during the summer of 1995. Herbicide degradation products were three of the four most frequently detected compounds for this study. The degradation product alachlor ethanesulfonic acid was the most frequently detected compound (65.1%), followed by atrazine (40.6%), and the degradation products deethylatrazine (34.9%), and cyanazine amide (19.8%). The corn herbicide acetochlor, first registered for widespread use in the United States in March 1994, was detected in a single water sample. No reported herbicide compound concentrations for this study exceeded currem U.S. Environmental Protection Agency's maximum contaminant levels or health advisory levels for drinking water, although the herbicide degradation products examined have yet to have such levels established. The occurrence of herbicide compounds had a significant, inverse relation to well depth and a significant, positive relation to dissolved‐oxygen concentration. It is felt that both well depth and dissolved oxygen are acting as rough surrogates to ground‐water age, with younger ground water being more likely to contain herbicide compounds. The occurrence of herbicide compounds was substantially different among the major aquifer types across Iowa, being detected in 82.5% of the alluvial, 81.8% of the bedrock/ karst region, 40.0% of the glacial‐drift, and 25.0% of the bedrock/nonkarst region aquifers. The observed distribution was partially attributed to variations in general ground‐water age among these aquifer types. A significant, inverse relation was determined between total herbicide compound concentrations in ground water and the average soil slope within a 2‐km radius of sampled wells. Steeper soil slopes may increase the likelihood of surface runoff occurring rather than ground‐water infiltration–decreasing the transport of herbicide compounds to ground water. As expected, a significant positive relation was determined between intensity of herbicide use and herbicide concentrations in ground water.

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Lake Houston is a major source of public water supply and recreational resource for the Houston metropolitan area, Texas. Water-quality issues of potential concern for the lake have included nutrient enrichment (orthophosphorus, total phosphorus, nitrite plus nitrate) and aquatic life use (dissolved oxygen). at Lake Houston sites showed no trend during 1990-2004; however, significant trends overall or for particular seasons, or both, were detected at some sites for nitrite plus nitrate, dissolved phosphorus, dissolved organic carbon, chlorophyll-a, and diazinon (2000-2004 data only for diazinon).

Water-quality trends were assessed in Lake Houston using age-dated sediment cores. Sediments deposited in the lake contain a partial chemical signature of human activities in the watershed. Over time, a water-quality history is recorded in the bottom sediments. Although the sediments in Lake Houston are clean compared to sediment-quality guidelines, increasing concentrations of mercury, zinc, and polycyclic aromatic hydrocarbons (PAHs) during the past several decades are evidence of the increasing human effect on water quality. The positive effects of regulation are indicated by decreases in concentrations of lead and DDT. As cities grow and urban sprawl replaces rangeland, agricultural fields, and forests, water quality declines (Van Metre and others, 2000). Large numbers of people, cars and trucks, the presence of industries, and large amounts of impervious surface all contribute to the decline. Conversely, new regulations and other changes in human activities, for example, banning a pesticide or switching to unleaded gasoline, can result in improvements in water quality (Callender and Van Metre, 1997; Van Metre and others, 1997). Many of the contaminants of concern in the environment are hydrophobic (literally "water fearing") and are, therefore, mostly attached to soils and sediments. Just because they are not dissolved in water, however, doesn't mean they are not of concern. In 2001 the Agency for Toxic Substances and Disease Registry and the U.S. Environmental Protection Agency (USEPA) published a priority list of hazardous substances (Agency for Toxic Substances and Disease Registry, 2001). Fifteen of the top 20 contaminants on the list are hydrophobic, including 4 trace elements and 11 organic contaminants. One method for determining trends in hydrophobic contaminants is the chemical analysis of sediment cores. As sediments accumulate in lakes and bays, they create a partial historical record of water quality. The purpose of this study was to determine historical trends in the concentrations of selected hydrophobic contaminants in the Lake Houston watershed using sediment cores from the lake. The study was a cooperative effort between the U.S. Geological Survey (USGS) and the City of Houston. This report describes the methods of collecting and age-dating sediment cores and the resulting historical trends in hydrophobic contaminants. The report also briefly describes the results of tributary streambed sampling to help explain differences in sources of contaminants to the lake on the basis of land use. The report concludes with implications of the findings for the water quality of Lake Houston. Lake Houston is a shallow reservoir on the San Jacinto River located about 25 miles northeast of Houston, Tex. It was constructed by the City of Houston in 1954 with a maximum capacity of 146,700 acre-feet and surface area of 19.1 square miles. The drainage area for Lake Houston (fig. 1) is 2,835 square miles. The upper end of the lake splits into two arms. Tributaries to the western arm include the West Fork San Jac...

As part of the Iowa GroundWater Monitoring Program (IGWM). water samples have been collected from selected Iowa municipal wells since 1982. An examination of this data identified two trends: (1) concentrations of atrazine in Iowa's ground water generally were decreasing over time, and (2) concentrations of metolachlor generally were increasing. Continuing groundwater sampling can determine if these trends represent long-term changes in chemical concentrations.