Enrichment of streams with nutrients is not simply explained by differences in land use. Land-management practices, nitrogen inputs to the land surface, local and regional environmental characteristics, and seasonal effects also control the degree of enrichment. Such an integration of factors explains why nutrient concentrations can be so different in different regions of the Nation, despite seemingly similar land-use settings.
Nutrient inputs to the land are key to explaining variations in the amount of nutrients lost from watersheds to streams (nutrient yields). The amounts of nutrients reaching streams generally increase as the total nonpoint nutrient inputs increase. For streams, nitrogen yields were less than or equal to about one-half of the total nonpoint inputs of nitrogen from the atmosphere,
|Average annual total nitrogen and total phosphorus yields to agricultural streams generally increase as nutrient inputs increase; however, a greater proportion of nitrogen than phosphorus input generally was lost to streams. The large amount of scatter in the data can be explained by local differences in agricultural practices, soils, geology, and hydrology across the Nation.|
Local watershed characteristics and environmental settings also play key roles in determining nutrient yields. Watersheds with high nitrogen yields compared to total nitrogen inputs include Bachman Run, East Mahantango Creek, Kishacoquillas Creek, and Muddy Creek in the Lower Susquehanna River Basin; Broad Brook in the Connecticut, Housatonic, and Thames River Basins; and Zollner Creek in the Willamette Basin. These agricultural streams generally are in areas of high precipitation, which enhances runoff of surface water and flushing of shallow ground water, along with nitrogen, to streams. These watersheds also have a long history of farming, and they are located where soils and underlying geologic formations allow rapid movement of nitrogen-rich water through shallow aquifers and into streams.
Characteristics of soil drainage can accentuate or mitigate nutrient yields to streams. For example, the Lost River in the White River Basin exhibited high nitrate and phosphorus yields, particularly during high-flow conditions. Despite relatively low nutrient inputs, high nutrient yields probably result from sloping, clayey soils and shallow depth to permeable karst bedrock, which allow rapid transport of nutrients to the Lost River. Watersheds with high nitrogen inputs and low nitrogen yields, such as Prairie and Shell Creeks in the Central Nebraska Basins, have relatively flat-lying, sandy or silty soils where water infiltrates readily and nitrate migrates to shallow ground water instead of being transported to streams.
Environmental factors controlling phosphorus yields to streams can be different from those controlling nitrogen yields. Watersheds with low phosphorus inputs and high phosphorus yields include Bullfrog Creek in the Georgia-Florida Coastal Plain--an unusual case reflecting contributions from naturally occurring phosphate minerals--and Broad Brook in the Connecticut, Housatonic, and Thames River Basins, which receives supplemental fertilization from phosphorus-rich manure. Basins with high phosphorus inputs and low yields include streams in the Lower Susquehanna River Basin, San Joaquin-Tulare Basins, and Albemarle-Pamlico Drainage. These streams gain significant flow from ground-water discharge, a source typically low in phosphorus because phosphates tend to be retained by the soil. Some of these same streams receive ground water that is high in nitrate because nitrogen inputs to the basins are high and nitrate can remain in solution. This is particularly true where denitrification, a microbial process that can transform nitrate to nitrogen gas, is not a controlling factor.
Local differences in soils, geology, and hydrology affect nitrate migration from nonpoint sources to ground water in a more pronounced way than for nutrient yields to streams. Inputs of nitrogen were estimated from atmospheric, commercial fertilizer, and manure sources for areas within a one-third-mile radius of each monitoring well. Study areas with low inputs of nitrogen and high median nitrate concentrations (greater than about 4 mg/L) generally are underlain by karst or fractured rock or by unconsolidated sand and gravel that allow nitrate to move readily to shallow ground water. Such areas are found in the San Joaquin-Tulare Basins, Central Columbia Plateau, Red River of the North Basin, Western Lake Michigan Drainages, Lower Susquehanna River Basin, Potomac River Basin, and Connecticut, Housatonic, and Thames River Basins.
Areas with high nitrogen inputs but low median nitrate concentrations (less than about 2 mg/L) generally are underlain by relatively impermeable rock, silt, or clay, which impede downward movement of water. Examples of these areas are found in the Rio Grande Valley, White River Basin, and Western Lake Michigan Drainages. The Jerome-Gooding agricultural site in the Upper Snake River Basin also fell in the high-input and low-concentration group, but this was more likely related to the deep water table (median of 153 feet) in this area.
Nutrient concentrations vary throughout the year, largely in response to changes in precipitation and streamflow and to differences in time since fertilizer or manure application. Nutrient concentrations in streams typically are elevated during high spring and summer streamflows, or peak irrigation periods, following fertilizer application. In two agricultural streams in the Western Lake Michigan Drainages, for example, more phosphorus was transported during storms in June 1993 than during the 24 months that followed. High nutrient concentrations also can be found in streams during seasonal low-flow conditions. Nitrate concentrations in agricultural streams can be high during winter low flow because of contributions from ground-water discharge and (or) because algal uptake is low. Nitrogen and phosphorus concentrations in streams downstream from metropolitan areas may be highest during various seasonal low flows, when contributions from point sources are greater relative to streamflow, and dilution is less.
Nitrate levels in shallow ground water can change throughout the year, but typically the seasonal changes are noticeable only in the upper 5-10 feet of the water table in surficial aquifers. For example, nitrate concentrations in shallow ground water from less than 10 feet below the water table in parts of the Red River of the North Basin ranged from about 8 to 25 mg/L from March 1994 through September 1995. This variation was related in part to the timing of important recharge periods, which generally occurred when spring snowmelt and major summer rainstorms coincided with irrigation periods, and in part to variations in the timing and application of fertilizers applied to crops.
Irrigation and agricultural drainage can play a major role in the timing and magnitude of nutrient concentrations, particularly in the western part of the Nation, where large fluctuations in streamflow occur because of diversions for irrigation. Return flows from agricultural land during the irrigation season can account for most of the flow in many western streams and rivers, and concentrations of potential contaminants often are highest during peak irrigation periods. In addition, low nutrient concentrations in irrigation canals can dilute concentrations in ground water in areas where direct connections occur between the canals and adjacent aquifers.
Stream-aquifer interactions can affect nutrient concentrations differently during different times of the year in the same river reach. For example, nitrate concentrations in shallow ground water adjacent to the lower Suwannee River in the Georgia-Florida Coastal Plain vary seasonally because of a cycle of water exchange between the river and the adjoining aquifer. During summer low flow, ground water containing high nitrate concentrations enters the river, increasing river nitrate concentrations. During spring high flow, river water low in nitrate enters the aquifer, resulting in a decrease in ground-water nitrate concentrations adjacent to the river.
Stream-aquifer interactions also can affect nutrient concentrations differently in different parts of the same river basin. For example, nitrate concentrations in about one-half of the wells sampled near the South Platte River in Colorado exceeded the USEPA drinking-water standard. Ground water contributes a substantial amount of flow to the river in this area, but concentrations of nitrate in the river were substantially lower than in ground water because microbial denitrification removed nitrate as ground water passed through the streambed. Farther downstream in Nebraska, ground water in the alluvial aquifer adjacent to the Platte River is used for public supply by Nebraska's largest cities, including Omaha, Lincoln, Grand Island, and Kearney. Pumping water from wells in this aquifer induces flow of Platte River water into the aquifer and has the potential to decrease nitrate concentrations in the ground water.
|Modeling integrates information to estimate risks of nitrate contamination to shallow ground water|
|Nutrient conditions differ by land use|
|The Quality of Our Nation's Water--Nutrients and Pesticides|