Surface and ground water in Nebraska may contain contaminants resulting from human activities. For purposes of this publication, a contaminant is any element or compound whose presence may affect the water's suitability for certain uses. For example, herbicide concentrations may exceeed the U.S. Environmental Protection Agency's (USEPA) Health Advisory Levels (HAL) for drinking water or trace-element concentrations may exceed guidelines for the protection of aquatic life. In general, the contaminats discussed in this report enter the aquatic system through nonpoint-source runoff from agricultural lands that dominate the Nebraska landscape. However,because this assessment was conducted as part of a larger, national program, a screening for contaminants with non-agricultural origins was included.

The measurement of water quality involves a variety of steps, each contributing unique information while also aggregating to an overall assessment. One aspect of water-quality assesment is to describe the occurrence and distribution of contaminants. Some contaminants may be hundreds or thousands of times more concentrated in the tissues of aquatic organisms or in fine sediments than they are in the water. As a result, fish tissue and streambed sediments are well suited for the detection of certain contaminants. For example, pesticides used in the United States prior to the early 1970's, such as DDT, may have degraded into more stable but still toxic compounds that are highly concentrated in fish tissues. Conversely, other contaminants are not concentrated in sediments or tissues but are readily detected in water samples. Organonitrogen herbicides (such as atrazine), the most commonly used herbicides in Nebraska, are examples of water-soluble contaminants.

Several sampling strategies were used to address specific questions. Some sites were sampled repeatedly through time and during all hydrologic conditions, whereas others were sampled only once to determine presence of contaminants. Because a strong relation between concentration and streamflow often exists for contaminants originating from nonpoint sources, streams typically were sampled near gaging stations that monitor streamflow.