Streams draining areas of extensive cropland (central and southern parts of the Study Unit) had the highest concentrations of nutrients (dissolved phosphorus, nitrate, and organic nitrogen) and the most detections of herbicides (Tornes and others, 1997). No stream water exceeded any of the existing drinking-water standards for pesticides or nutrients.
Table 2 shows that selected indicators of water quality varied across the four subregions, which generally relate to major categories of land use (fig. 2). The water-quality indicators used are commonly associated with agricultural land use but are also affected by environmental conditions, such as geology, soils, climate and hydrology.
|Water-quality||Water||Subregion||Remarks or specific findings|
|Mean total, (mostly||SW||0.36||0.17||--||0.23||Central does not include the Red River as an indicator of this subregion.|
|herbicides)||GW||.01||.05||--||.03||Southeast had highest percent detection rate, but values were all low.|
|Median total nitrogen||SW||.67||1.02||0.82||.59||The Red River is a major source of nutrients to Lake Winnipeg.|
|Median nitrate nitrogen||GW||.39||<.05||--||<.05||Nitrate concentrations in ground water exceeded the USEPA drinking-water standard in 27, 0, and 8 percent of samples in the west, central, and southeast subregions, respectively.|
|Median total phosphorus||SW||.14||.14||.04||.03||Minnesota Pollution Control Agency goal is 0.10 mg/L phosphorus in rivers.|
|Median orthophosphate (dissolved)||GW||.03||.01||--||.02||Subregions with high nutrient levels do not necessarily have high pesticide levels.|
|Median Index of Biotic Integrity (IBI)||SW||41||25||36||43||These scores were strongly related to stream habitat.|
|Pesticide||Annual rate of application (1990)||Load at Emerson (1993–95)||Percent output||Atrazine||120,000||1,100||0.9|
The most heavily applied pesticides (2,4-D, MCPA, bromoxynil, and trifluralin) were not always the most frequently detected in streams and shallow aquifers (fig. 7 and tables 6 and 7, p. 24-26) (Tornes and others, 1997; Cowdery, in press). The infrequency of 2,4-D detection may be related to factors such as the following: (1) 2,4-D is applied as a post-emergent herbicide and mostly is taken up by plants where it is metabolized to other compounds, (2) soil microbes effectively degrade 2,4-D, or (3) soils retain 2,4-D instead of allowing it to run off or seep downward into ground water (Tornes and others, 1997). Triallate was detected in northern streams of the Study Unit, an area where it is most commonly applied to small grains. Triallate usually was applied during autumn and reached streams during spring snowmelt runoff.
(14,755 bytes) Figure 7. More pesticides were detected in streams than in shallow surficial ground water, and most were herbicides.
Figure 7. More pesticides were detected in streams than in shallow surficial ground water, and most were herbicides.
(17,010 bytes) Figure 8. Cropland applications of nitrogen and
phosphorus have contributed nutrients to streams (1993-95).
(6,090 bytes) Figure 9. Nitrate concentrations in streams differed by subregion in the Study Unit.
Figure 8. Cropland applications of nitrogen and phosphorus have contributed nutrients to streams (1993-95).The median concentration of nitrate was higher in the Red River than in the tributaries (fig. 9). These values indicate that activities in and near the Red River are contributing to the nitrate concentrations in the river. These activities could be both agricultural and nonagricultural.
Figure 9. Nitrate concentrations in streams differed by subregion in the Study Unit.Generally, water in aquifers sampled for the Red River Basin Study Unit is safe to drink relative to nutrients and herbicides (Cowdery, in press) (fig. 10). As related to agricultural pesticide application, volatile organic compounds (VOCs) were not detected in surficial aquifers (Cowdery, 1997; Cowdery, in press). Concentrations of pesticides and nutrients in water from the buried aquifers, naturally protected by overlying sediments, indicated no significant contamination from irrigation (Cowdery, in press).
(9,652 bytes) Figure 10. Agricultural chemical concentrations in ground water differed by subregion in the Study Unit.
Figure 10. Agricultural chemical concentrations in ground water differed by subregion in the Study Unit.
Nitrate concentrations near the water table exceeded the drinking-water standard (10 mg/L) in some areas but decreased significantly at greater depths in the surficial aquifers (Cowdery, 1997). Irrigation has enhanced crop production in some areas by allowing for increased yields and a greater variety of crops. Increased applications of fertilizer and pesticides are sometimes associated with irrigation. Irrigation has also been associated with pesticides and nutrients reaching parts of some surficial aquifers. The concentrations of pesticides in shallow ground water in the irrigated parts of the Otter Tail outwash aquifer (fig. 11) are higher than elsewhere in the Study Unit and indicate the potential for contamination in deeper ground water (Cowdery, 1997).
Figure 11. Several sites of possible stream-aquifer interaction occur in the Red River of the North Basin Study Unit.A detailed study of the Otter Tail outwash aquifer (see page 22) that included analyses of ground-water age, water chemistry, and historical land use showed a trend of increased nitrate and nitrogen levels over the past 30 years (Stoner and others, 1997). The combination of irrigation, sandy soils, aquifer materials with low carbon content, and conditions not favorable for biological reduction of nitrate resulted in elevated nitrate in the shallow ground water.
The Otter Tail outwash aquifer, with high nitrate concentrations near the water table, discharged water with low nitrate concentrations to the Otter Tail River, according to one intensive case study (Puckett and others, 1995; Tornes and others, 1996; Stoner and others, 1997). Mixing with older, low-nitrate ground water and denitrification (whereby nitrate was transformed to nitrogen gas and removed from the water) as water flowed beneath riparian wetlands accounted for the low concentrations of nitrate discharging to the river. These conditions helped reduce the possibility of eutrophication in the river and downstream lakes and wetlands. Figure 11 shows other areas in the Study Unit where surficial aquifers potentially discharge water to streams.
Fish distribution and abundance may not be a good indicator of nutrient effects on stream quality. Most of the nutrients enter the stream early in the spring when temperatures are low and most metabolic rates for aquatic plants and animals likewise are low. Therefore, the amount of nutrients applied in a watershed correlated poorly with fish distribution and abundance.