Table 7-1. Effectiveness of Wetlands and Riparian Areas for NPS Pollution Control

Riparian Forests

This study looks at how various soil types affect the buffer width necessary for effectiveness of riparian forests to reduce loadings of agricultural nonpoint source pollutants.

• A hypothetical buffer with a width of 30 m and designed to remove 90% of the nitrate nitrogen from runoff volumes typical of 50 acres of row crop on relatively poorly drained soils was used as a standard.

• Udic upland soils and sandy entisols met or exceeded these standards.

• The study also concluded that slope gradient was the most important contributor to the variation in effectiveness. Phillips, J.D. 1989. Nonpoint Source Pollution Control Effectiveness of Riparian Forests Along a Coastal Plain River. Journal of Hydrology, 110 (1989):221-237.

2 - Lake Tahoe, Nevada

Riparian

Three years of research on a headwaters watershed has shown this area to be capable of removing over 99% of the incoming nitrate nitrogen. Wetlands and riparian areas in a watershed appear to be able to "clean up" nitrate-containing waters with a very high degree of efficiency and are of major value in providing natural pollution controls for sensitive waters. Rhodes, J., C.M. Skau, D. Greenlee, and D. Brown. 1985. Quantification of Nitrate Uptake by Riparian Forests and Wetlands in an Undisturbed Headwaters Watershed. In Riparian Ecosystems and Their Management: Reconciling Conflicting Issues. USDA Forest Service GTR RM-120, pp. 175-179.

3 - Atchafalaya, Louisiana

Riparian

Overflow areas in the Atchafalaya Basin had large areal net exports of total nitrogen (predominantly organic nitrogen) and dissolved organic carbon but acted as a sink for phosphorus. Ammonia levels increased dramatically during the summer. The Atchafalaya Basin floodway acted as a sink for total organic carbon mainly through particulate organic carbon (POC). Net export of dissolved organic carbon was very similar to that of POC for all three areas. Lambou, V.W. 1985. Aquatic Organic Carbon and Nutrient Fluxes, Water Quality, and Aquatic Productivity in the Atchafalaya Basin, Louisiana. In Riparian Ecosystems and Their Management: Reconciling Conflicting Issues. USDA Forest Service GTR RM-120, pp. 180-185.

4 - Wyoming

Riparian

The Green River drains 12,000 mi2 of western Wyoming and northern Utah and incorporates a diverse spectrum of geology, topography, soils, and climate. Land use is predominantly range and forest. A multiple regression model was used to associate various riparian and nonriparian basin attributes (geologic substrate, land use, channel slope, etc.) with previous measurements of phosphorus, nitrate, and dissolved solids. Fannin,T.E., M. Parker, and T.J. Maret. 1985. Multiple Regression Analysis for Evaluating Non-point Source Contributions to Water Quality in the Green River, Wyoming. In Riparian Ecosystems and Their Management: Reconciling Conflicting Issues. USDA Forest Service GTR RM-120, pp. 201-205.

5 - Rhode River Subwater-shed, Maryland

Riparian

A case study focusing on the hydrology and below-ground processing of nitrate and sulfate was conducted on a riparian forest wetland. Nitrate and sulfate entered the wetland from cropland ground-water drainage and from direct precipitation. Data collected for 3 years to construct monthly mass balances of the fluxes of nitrate and sulfate into and out of the soils of the wetland showed:

• Averages of 86% of nitrate inputs were removed in the wetland.
• Averages of 25% of sulfates were removed in the wetland.
• Annual removal of nitrates varied from 87% in the first year to 84% in the second year.
• Annual removal of sulfate varied from 13% in the second year to 43% in the third year.
• On average, inputs of nitrate and sulfate were highest in the winter.
• Nitrate outputs were always highest in the winter.
• Nitrate removal was always highest in the fall (average of 96%) when input fluxes were lowest and lowest in winter (average of 81%) when input fluxes were highest. Correll, D.L., and D.E. Weller. 1989. Factors Limiting Processes in Freshwater: An Agricultural Primary Stream Riparian Forest. In Freshwater Wetlands and Wildlife, ed. R.R. Sharitz and J.W. Gibbons, pp. 9-23. U.S. Department of Energy, Office of Science and Technology, Oak Ridge, Tennessee. DOE Symposium Series #61.

Riparian

Ground water is closely coupled with streamflow to maintain water supply to riparian vegetation, particularly where precipitation is seasonal. A case study is presented where Mediterranean climate and ground-water extraction are linked with the decline of riparian vegetation and subsequent severe bank erosion on the Carmel River. Groenveld, D. P., and E. Griepentrog. 1985. Interdependence of Groundwater, Riparian Vegetation, and Streambank Stability: A Case Study. In Riparian Ecosystems and their Management: Reconciling Conflicting Issues. USDA Forest Service GTR RM-120, pp. 201-205.

7 - Cashe River, Arkansas

Riparian

A long-term study is being conducted to determine the chemical and hydrological functions of bottomland hardwood wetlands. Hydrologic gauging stations have been established at inflow and outflow points on the river, and over 25 chemical constituents have been measured. Preliminary results for the 1988 water year indicated:

• Retention of total and inorganic suspended solids and nitrate;
• Exportation of organic suspended solids, total and dissolved organic carbon, inorganic carbon, total phosphorus, soluble reactive phosphorus, ammonia, and total Kjeldahl nitrogen;
• All measured constituents were exported during low water when there was limited contact between the river and the wetlands; and
• All measured constituents were retained when the Cypress-Tupelo part of the floodplain was inundated. Kleiss, B. et al. 1989. Modification of Riverine Water Quality by an Adjacent Bottomland Hardwood Wetland. In Wetlands: Concerns and Successes, pp. 429-438. American Water Resources Association.

Riparian

Nitrate removal in riparian areas was determined using a mass balance procedure in a small New Zealand headwater stream. The results of 12 surveys showed:

• The majority of nitrate removal occurred in riparian organic soils (56-100%) even though the soils occupied only 12% of the stream's border.
• The disproportionate role of organic soils in removing nitrate was due in part to their location in the riparian zone. A high percentage (37-81%) of ground water flowed through these areas on its passage to the stream.
• Anoxic conditions and high concentrations of denitrifying enzymes and available carbon in the soils also contributed to the role of the organic soils in removing nitrates. Cooper, A.B. 1990. Nitrate Depletion in the Riparian Zone and Stream Channel of a Small Headwater Catchment. Hydrobiologia, 202:13-26.

Riparian

Changes in nitrate concentrations in ground water between an agricultural field planted in tall fescue (Festuca arundinacea) and riparian zones vegetated by leguminous or nonleguminous trees were measured to:

• Determine the effectiveness of riparian vegetation management practices in the reduction of nitrate concentrations in ground water;
• Identify effects of leguminous and nonleguminous trees on riparian attenuation of nitrates; and
• Measure the seasonal variability of riparian vegetation's effect on the chemical composition of ground water.

• Ground-water nitrate concentrations beneath non-leguminous riparian trees decreased toward the shoreline, and removal of the trees resulted in increased nitrate concentrations.

• Nitrate concentrations did not decrease from the field to the riparian zone in ground water below leguminous trees, and removal of the trees resulted in decreased ground-water nitrate concentrations.

• Maximum attenuation of nitrate concentrations occurred in the fall and winter under non-leguminous trees. James, B.R., B.B. Bagley, and P.H. Gallagher, P.H. 1990. Riparian Zone Vegetation Effects on Nitrate Concentrations in Shallow Groundwater. Submitted for publication in the Proceedings of the 1990 Chesapeake Bay Research Conference. University of Maryland, Soil Chemistry Laboratory, College Park, Maryland.

10 - Little Lost Man Creek, Humboldt, California

Riparian

Nitrate retention was evaluated in a third-order stream under background conditions and during four intervals of modified nitrate concentration caused by nutrient amendments or storm-enhanced discharge. Measurements of the stream response to nitrate loading and storm discharge showed:

• Under normal background conditions, nitrate was exported from the subsurface (11% greater than input).
• With increased nitrate input, there was an initial 39% reduction from the subsurface followed by a steady state reduction of 14%.
• During a storm event, the subsurface area exported an increase of 6%. Triska, F.J., V.C. Kennedy, R.J. Avanzino, G.W. Zellweger, and K.E. Bencala. 1990. In Situ Retention-Transport Response to Nitrate Loading and Storm Discharge in a Third-Order Stream. Journal of North American Benthological Society, 9(3):229-239.

Riparian

Field enrichments of nitrate in two spring-fed drainage lines showed an absence of nitrate depletion within the riparian zone of a woodland stream. The results of the study indicated:

• The efficiency of nitrate removal within the riparian zone may be limited by short water residence times.
• The characteristics of the substrate and the routes of ground-water movement are important in determining nitrate attenuation within riparian zones. Warwick, J., and A.R. Hill. 1988. Nitrate Depletion in the Riparian Zone in a Small Woodland Stream. Hydrobiologia, 157:231-240.

A study was conducted on riparian forests located adjacent to agricultural uplands to test their ability to intercept and utilize nutrients (N, P, K, Ca) transported from these uplands. Tissue nutrient concentrations, nutrient accretion rates, and production rates of woody plants on these sites were compared to control sites. Data from this study provide evidence that young (bloom state) riparian forests within agricultural ecosystems absorb nutrients lost from agricultural uplands. Fail, J.L. Jr., Haines, B.L., and Todd, R.L. Undated. Riparian Forest Communities and Their Role in Nutrient Conservation in an Agricultural Watershed. American Journal of Alternative Agriculture, II(3):114-120.

13 - Chowan River Watershed, North Carolina

Riparian

A study was conducted to determine the trapping efficiency for sediments deposited over a 20-year period in the riparian areas of two watersheds. 137CS data and soil morphology were used to determine areal extent and thickness of the sediments. Results of the study showed:

• approximately 80% of the sediment measured was deposited in the floodplain swamp.
• Areater than 50% of the sediment was deposited within the first 100 m adjacent to cultivated fields.
• aediment delivery estimates indicated that 84% to 90% of the sediment removed from cultivated fields remained in the riparian areas of a watershed. Cooper, J.R., J.W. Gilliam, R.B. Daniels, and W.P. Robarge. 1987. Riparian Areas as Filters for Agriculture Sediment. Soil Science Society of America Journal, 51(6):417-420.

Riparian

Several recent studies in agricultural fields and forests showed evidence of significant nitrate removal from drainage water by riparian zones. The results of these studies showed:

• d typical removal of nitrate of greater than 85% and
• dn increase of nitrate removal by denitrification where greater contact occurred between leaching nitrate and decaying vegetative matter. Schipper, L.A., A.B. Cooper, and W.J. Dyck. 1989. Mitigating Non-point Source Nitrate Pollution by Riparian Zone Denitrification. Forest Research Institute, Rotorua, New Zealand.

Riparian

A streamside, mixed hardwood, riparian forest near Tifton, Georgia, set in an agricultural watershed was effective in retaining nitrogen (67%), phosphorus (25%), calcium (42%), and magnesium (22%). Nitrogen was removed from subsurface water by plant uptake and microbial processes. Riparian land use was also shown to affect the nutrient removal characteristics of the riparian area. Forested areas were more effective in nutrient removal than pasture areas, which were more effective than croplands. Lowrance, R.R., R.L. Todd, and L.E. Asmussen. 1983. Waterborne Nutrient Budgets for the Riparian Zone of an Agricultural Watershed. Agriculture, Ecosystems and Environment, 10:371-384.

16 - North Carolina

Riparian

Riparian forests are effective as sediment and nutrient (N and P) filters. The optimal width of a riparian forest for effective filtering is based on the contributing area, slope, and cultural practices on adjacent fields. Cooper, J. R., J. W. Gilliam, and T. C. Jacobs. 1986. Riparian Areas as a Control of Nonpoint Pollutants. In Watershed Research Perspectives, ed. D. Correll, Smithsonian Institution Press, Washington, DC.

17 - Unknown

Riparian

A riparian forest acted as an efficient sediment trap for most observed flow rates, but in extreme storm events suspended solids were exported from the riparian area. Karr, J.R., and O.T. Gorman. 1975. Effects of Land Treatment on the Aquatic Environment. In U.S. EPA Non-Point Source Pollution Seminar, pp. 4-1 to 4-18. U.S. Environmental Protection Agency, Washington, DC. EPA 905/9-75-007.

18 - Arkansas

Riparian

The Army Corps of Engineers studied a 20-mile stretch of the Cashe River in Arkansas where floodplain deposition reduced suspended solids by 50%, nitrates by 80%, and phosphates by 50%. Stuart, G., and J. Greis. 1991. Role of Riparian Forests in Water Quality on Agricultural Watersheds.

19 - Maryland

Riparian

Phosphorus export from the forest was nearly evenly divided between surface runoff (59%) and ground-water flow (41%), for a total P removal of 80%. The mean annual concentration of dissolved total P changed little in surface runoff. Most of the concentration changes occurred during the first 19 m of the riparian forest for both dissolved and particulate pollutants. Dissolved nitrogen compounds in surface runoff also declined. Total reductions of 79% for nitrate, 73% for ammonium-N and 62% for organic N were observed. Changes in mean annual ground-water concentrations indicated that nitrate concentrations decreased significantly (90-98%) while ammonium-N concentrations increased in concentration greater than threefold. Again, most of the nitrate loss occurred within the first 19 m of the riparian forest. Thus it appears that the major pathway of nitrogen loss from the forest was in subsurface flow (75% of the total N), with a total removal efficiency of 89% total N. Peterjohn, W.T., and D.L. Correll. 1984. Nutrient Dynamics in an Agricultural Watershed: Observations on the Role of a Riparian Forest. Ecology, 65:1466-1475.

20 - France

Riparian

Denitrification explained the reduction of the nitrate load in ground water beneath the riparian area. Models used to explain the nitrogen dynamics in the riparian area of the Lounge River indicate that the frequency, intensity, and duration of flooding influence the nitrogen-removal capacity of the riparian area.

Three management practices in riparian areas would enhance the nitrogen-removal characteristics, including:

• fiver flow regulation to enhance flooding in riparian areas, which increases the waterlogged soil areas along the entire stretch of river;

• feduced land drainage to raise the water table, which increases the duration and area of waterlogged soils; and

• fecreased deforestation of riparian forests, which maintains the amount of carbon (i.e., the energetic input that allows for microbial denitrification). Pinay, G., and H. Decamps. 1988. The Role of Riparian Woods in Regulating Nitrogen Fluxes Between the Alluvial Aquifer and Aurface Water: A Conceptual Model. Regulated Rivers: Research and Management, 2:507-516.

21 - Georgia

Riparian

Processes within the riparian area apparently converted primarily inorganic N (76% nitrate, 6% ammonia, 18% organic N) into primarily organic N (10% nitrate, 14% ammonia, 76% organic N). Lowrance, R.R., R.L. Todd, and L.E. Assmussen. 1984. Nutrient Cycling in an Agricultural Watershed: Phreatic Movement. Journal of Environmental Quality, 13(1):22-27.

22 - North Carolina

Riaprian

Subsurface nitrate leaving agricultural fields was reduced by 93% on average. Jacobs, T.C., and J.W. Gilliam. 1985. Riparian Losses of Nitrate from Agricultural Drainage Waters. Journal of Environmental Quality, 14(4):472-478.

23 - North Carolina

Riparian

Over the last 20 years, a riparian forest provided a sink for about 50% of the phosphate washed from cropland. Cooper, J.R., and J.W. Gilliam. 1987. Phosphorus Redistribution from Cultivated Fields into Riparian Areas. Soil Science Society of America Journal, 51(6):1600-1604.

24 - Illinois

Riparian

Small streams on agriculture watersheds in Illinois had the greatest water temperature problems. The removal of shade increased water temperature 10-15 degrees Fahrenheit. Slight increases in water temperature over 60 øF caused a significant increase in phosphorus release from sediments. Karr, J.R., and I.J. Schlosser. 1977. Impact of Nearstream Vegetation and Stream Morphology on Water Quality and Stream Biota. Ecological Research Series, EPA-600/3-77-097. U.S. Environmental Protection Agency, Washington, DC.

This page last updated October 4, 1999