Previous sections of this report have examined the effects of different land uses on water quality using a combination of physical, chemical, and biological information from streams and aquifers. In this section, the status of water quality for each of four land uses will be compared and contrasted, and the role of land use as the primary factor in determining water quality will be discussed. A determination of the relative status of water quality will be accomplished by aggregating NAWQA data by land use and then summarizing the results in a single graphical display. Median values are used to characterize each land use, which places a greater emphasis on average characteristics for a land use rather than on extreme values that might occur at individual sites. The graphical display consists of nine measures of water quality for surface water and six measures of water quality for ground water. The surface-water measures include information on water, but also include information about stream habitat, fish communities, and suspended sediment. Each measure is ranked as to whether it is significantly affected, moderately affected, or minimally affected by land use. Details on how the 15 measures were classified and ranked are shown in the chart on the following page.
In the forested mountains, most measures rank as minimally affected by land use, but there are six measures that rank as more than minimally affected. Of these, four are affected by factors other than the dominant land use: trace elements in surface water are affected by mining, radon in ground water is affected by bedrock geology, and nitrate and VOCs in ground water are affected by low-density residential development. These exceptions indicate that other factors (such as geology) and minor land uses can have a larger effect on water quality than the dominant land use.
In the forested mountains, the habitat index (HDI) and fish index (IBI) also are ranked as moderately affected by land use. However, these two measures are ranked as moderately affected in all of the land uses. This situation arises because in each land use, there are sites that rank as minimally affected and sites that rank as more significantly affected; that is, there is as much variability among sites within the same land use as there is variability in sites between land uses. Habitat HDI and fish IBI can be affected by localized channel conditions as well as the land use in the immediate vicinity of the site.
The urban and mixed (urban/agriculture) land uses are very similar in overall water quality, with almost all measures ranked as moderately affected. There is considerable site-to-site variability in surface-water quality within urban land use, depending primarily on the magnitude of WWTP discharges near a site (Litke and Kimbrough, 1998). There is less variability in surface-water quality within the mixed (urban/agriculture) land use because sites in this land use integrate land-use effects. This integration was particularly evident for pesticides: concentrations of organochlorine pesticides and PCBs in fish were higher in mixed land use than elsewhere; and the largest variety of pesticides detected at a site occurred at a mixed (urban/agriculture) site. The significantly affected rankings for trace elements in surface water and radon in ground water within the forested land use do not carry through to the urban or mixed (urban/agricultural) land use. Much of the surface water from mountain streams is removed before it reaches the plains, and the crystalline mountain aquifers and plains alluvial aquifers are not hydrologically connected.
Agricultural land use has the most significant relative effect on water quality because it is a source for three constituents: nitrate in ground water, specific conductance (salinity) in surface water and ground water, and suspended sediment in surface water. Ground water in the agricultural land use is derived almost entirely from irrigation return flows, which infiltrate into the aquifer. Therefore, ground water is very representative of agricultural land use: it is highly affected by nitrate and salinity, but minimally affected by VOCs and radon. Surface water in the agricultural area, however, has three primary sources: upstream urban water, surface return flows from agricultural lands (irrigation return flows and occasional storm runoff), and ground-water recharge from the aquifer. When upstream urban water predominates, urban contaminants are carried to the agricultural users. When irrigation return flows predominate, water quality is related primarily to agricultural land use. But the mix of sources leads to considerable geographic and temporal variability in agricultural water quality.
An integrated assessment of water-quality conditions using multiple lines of evidence is a valuable tool for examining the physical, chemical and biological factors affecting surface- and ground-water resources in the basin. Results from the NAWQA study demonstrate that natural and human factors can affect the quality of ground water and surface water and the biological community at a given site. Moreover, water quality can vary depending on the physical features (mountains compared to plains), hydrologic conditions, land use in the immediate vicinity of the site, water-management practices, and the land use upstream/upgradient from the site.
1 Classification breakpoints are 25th and 75th percentiles of data sets. For trace elements, exceedances are above background levels determined by Heiny and Tate, 1997.
2 Classification breakpoints were selected to bracket mean land-use concentrations.
3 Classification breakpoints modified from Schrader, 1989.