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Defining background conditions of water quality is important for water and land managers in assessing the effects of human activities, such as land use, on water resources. The background dissolved solids in rivers and ground water in the Red River Basin Study Unit include the major cations (calcium, magnesium, sodium, and potassium), the major anions (bicarbonate, chloride, and sulfate), trace elements (including iron and manganese), and radionuclides (uranium, radium, and radon). The most common ions in ground water from surficial and buried glacial aquifers are calcium, magnesium, and bicarbonate, all of which are at fairly low concentrations. Median dissolved solids are about 400 milligrams per liter (mg/L) for surficial glacial aquifers and 500 mg/L for buried glacial aquifers. Common ions in deeper bedrock aquifers are sodium and chloride (the components of table salt) and are at much higher concentrations (median dissolved-solids concentration about 1,900 mg/L).
Ground-water studies in the Study Unit demonstrated that water quality in surficial aquifers in the west and central subregions is significantly different than that in the southeast subregion (Cowdery, in press). These differences are in the concentrations of dissolved solids, sodium, sulfate, silica, potassium, uranium, and radium. The west and central subregions have higher concentrations of all of these ions except radium, which is higher in the east. Variations in water quality are related to natural differences in geology and hydrology. Saline sedimentary bedrock aquifers exist mostly in the western part of the Study Unit (fig. 6). Most of the eastern part is underlain by crystalline rocks that do not readily transmit water and were not considered for this study. The sedimentary bedrock aquifers slope gently upward to the east, in the direction of regional ground-water flow. Saline water from these aquifers is primarily discharged in the north-central part of the Study Unit. Saline ground water from deep aquifers seeping into some shallow buried and surficial aquifers in the west and central subregions can affect the quality and use of water in these aquifers. This saline ground water also can discharge into streams and degrade water quality in the northwestern part of the Study Unit. This effect can be greatest during periods of extremely low streamflow.
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Figure 6. Surficial aquifers are in glacial sediments throughout the Study Unit. Saline ground water in sedimentary rocks can seep to the land surface.
| U.S. Environmental | Percentage of ground-water samples | |||||
|---|---|---|---|---|---|---|
| Protection Agency | exceeding standard or advisory | Constituent | standards and health advisories | Glacial aquifer type | Subregion | |
| Buried | Surficial | West | Central | Southeast | ||
| Dissolved solids, in mg/L | 500 | 52 | 23 | 33 | 38 | 0 |
| Sodium, in mg/L | ¹20 | 82 | 21 | 27 | 33 | 4 |
| Chloride, in mg/L | 250 | 9 | 0 | 0 | 0 | 0 |
| Sulfate, in mg/L | 500 | 6 | 3 | 7 | 4 | 0 |
| Sulfate, in mg/L | 250 | 12 | 5 | 7 | 7 | 0 |
| Fluoride, in mg/L | ²2 | 0 | 0 | 0 | 0 | 0 |
| Iron, in mg/L | 0.3 | 82 | 48 | 13 | 52 | 64 |
| Manganese, in mg/L | 0.05 | 82 | 79 | 80 | 85 | 72 |
| Uranium, in mg/L | ³0.02 | 0 | 5 | 7 | 8 | 0 |
| Radium, in mg/L | ³0.005 | 0 | 0 | 0 | 0 | 0 |
| Radon, in pCi/L | 300 | 50 | 66 | 60 | 64 | 64 |
¹Health advisory
²Under review
³Prposed
Fish communities and stream habitat can be indicators of overall stream quality. For example, greater fish species diversity and abundance coincide with higher quality streams. Fish diversity and abundance in the streams of the Study Unit are influenced by human and natural factors (Goldstein and others, 1996b). Three factors explain about 60 percent of the variability in fish distribution: (1) the abundance and diversity of fish habitat within a stream, (2) the variability in the amount of water in the stream, and (3) the amount of relatively undisturbed land (forest or wetland) within about a mile of a stream. Habitat and stream variability are mostly natural factors, although both are influenced by human activities. Other factors considered in this study, such as number of dams, amount of drainage ditches, or width of riparian buffer zones, appear to have less effect on variability in fish distribution and abundance (Goldstein and others, 1996b).
