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Critical Areas in Agricultural Nonpoint Source Pollution Control Projects
The Rural Clean Water Program Experience

A primary objective of a nonpoint source (NPS) pollution control watershed project is to protect or restore the designated use of a water resource by reducing pollutant delivery to the water resource. Because nonpoint sources of pollution are usually widespread, intermittent, and undefined, mitigating a water quality problem, or potential problem, caused by NPS pollution is often difficult. The task is further complicated when sufficient time and funding are not available to implement all the recommended best management practices (BMPs). For this reason, a land treatment strategy should be developed to guide the selection and implementation of BMPs. While strategies can vary widely depending on hydrologic, sociologic, and agronomic factors, a key component of the most effective strategies is identification and appropriate treatment of NPS areas contributing disproportionately to the water quality problem. Concentrating land treatment efforts on these critical areas, or sources, helps ensure that available resources are appropriated as efficiently as possible.

All Rural Clean Water Program (RCWP) projects were required to identify and treat critical areas. However, since explicit guidance was not provided, project critical area criteria varied widely from simply all land within a set distance from a water resource to a complex set of factors applied to individual farms. The experiences of the 21 RCWP projects provide the basis for the following discussion of critical areas.

Reasons to Identify Critical Areas

All nonpoint sources of pollution are not equal. Many nonpoint sources of pollution are insignificant, while other sources contribute substantially to water resource impairment. Topographic, hydrologic, and agronomic factors often combine to make some nonpoint sources more detrimental to the beneficial use of water resources than others. Therefore, a method or strategy to identify and prioritize for treatment NPS areas that are more detrimental than others is desirable. Identifying and treating in order of priority the sources that most adversely affect the water resource help speed up the restoration process and may save time and money by achieving the same pollutant reduction by treating fewer sources.

Important Factors in Identifying and Defining Critical Areas

Hydraulic Transport of Pollutants to Water Resource of Concern


Defining critical NPS areas involves identifying the major pollutant sources and assessing the hydrologic transport system from the source to the water resource of concern. The purpose of this assessment is to estimate how much of the pollutant(s) of concern will actually affect the water resource. For example, if a pollutant source is on a small intermittent tributary that is slow moving and drains through a large wetland and several miles of stream before emptying into a lake, then the pollutant source may not be as critical as a similar source on a perennial stream within a mile of the lake. The difference in the efficiency of the hydrologic transport system makes the delivery of pollutants to the lake from one source much more likely than from the other. Therefore, although both pollutant sources should be treated, the source closer to the lake should be considered a higher priority.

The transport mechanisms by which pollutants are carried to a water resource help identify critical pollutant sources. Pollutants that are sorbed to sediment or organic matter are much less likely to be delivered to the water resource (because of settling or filtering en route) than are pollutants in the dissolved phase, such as nitrate.

Magnitude of the Pollutant Source


Another factor in determining critical sources is the magnitude of the source. A source area that contributes large amounts of pollutants to a waterway often has a significant impact on a water resource, regardless of the efficiency of the hydrologic system. A few sources of large amounts of pollutants can overload the filtering capability of natural waterways or, for ground water, overlying soils, thereby creating chronic water quality problems. Thus, the magnitude of the source as well as the hydrologic transport system must be considered in determining whether the source is critical.

Type of Pollutant


Finally, the type of pollutant must be considered in critical area selection. Examples of pollutants include: fine sediment that causes turbidity, larger sediment that causes reduced reservoir-storage capacity, phosphorus that causes eutrophication, and microbial pathogens or pesticides that cause health risks. Identifying the nprimary pollutant(s) facilitates focusing of land treatment on the critical sources causing the water quality impairment. For instance, a single pollutant may be the primary cause of the impairment; therefore, it would not be necessary to treat source areas of other pollutants. For example, when bacteria are causing the impairment, critical areas need include only those areas in which bacteria is a problem, such as in and around animal operations.

Type of Water Resource


Critical areas are also determined by the type of water resource that is impaired. Critical areas for ground water versus surface water problems may differ because the pollutants causing the impairment, sources of pollutants delivered to the water resource, and hydrology of the recharge area or watershed are different.

In the Minnesota RCWP project, the ground water recharge area was significantly different from the surface watershed and critical areas. The surface water resource, Garvin Brook, was a trout stream impaired by high sediment and nutrient loads. The surface watershed critical area was determined by distance to flowing water, sinkholes, and abandoned wells, then refined using the Agricultural Non-Point-Source Pollution Model (AGNPS) (Young et al., 1987). The impaired ground water resource was a shallow aquifer. The recharge area for the aquifer (critical area) extended outside the Garvin Brook watershed. Thus, source areas critical to one type of water resource may not be critical to another.

Severity and Type of Water Quality Problem


In general, the more severe the water quality problem, the greater the pollutant reduction and extent of land treatment required to reverse the problem. Also, the type of problem affects critical area selection. If peak concentrations or standard violations are the problem, critical areas may be determined by the maximum pollutant delivery rate. For example, surface drinking water supply impairments are often caused by peak pollutant concentrations. Conversely, critical area selection that minimizes pollutant accumulation can be important for addressing loss of reservoir storage capacity or destruction of benthic habitat.

Methods Used to Identify Critical Areas

Developing a set of scientific criteria facilitates systematic assessment of the factors involved in critical area identification. The ideal criteria incorporate the efficiency of the hydrologic system in pollutant transport, magnitude of the source, and type of pollutant into guidelines that can be applied throughout the watershed. However, due to the complexity of the task, criteria are often simplified to the point that they are of little value. One simple criterion used in the RCWP was to define as critical all cropland within a quarter mile of the water resource. This criterion ignored land use activities and potential pollutant loading from sources along tributaries. Another simple criterion, identifying the entire watershed as critical, is usually not feasible or efficient unless the project area is small and major pollutant sources are uniformly widespread. Pollutant transport, source magnitude, and pollutant type should each be addressed by the simplest set of criteria.

Critical area criteria for watersheds with animal waste problems are particularly complex because these watersheds often include two or more pollutants and various types of sources, such as land application of animal waste, untreated feedlots, and livestock lounging in streams. Criteria were used by several RCWP projects with animal waste problems. The presence of an untreated feedlot combined with its distance from a watercourse (column 2) was the criterion used most often to identify a critical pollutant source. Obviously, this criterion is important because untreated feedlots are sources of large quantities of nutrients and bacteria, and if untreated feedlots are near waterways, the probability of pollutant loading to the drainage system is high.

If this table looks garbled, click here .

Table 1. Major Criteria Used By RCWP Projects With Significant Animal Waste Problems.
RCWP Project Major Critical Area Criteria Degree of Adherence to Criteria
Feedlots without Treatment* Feedlot Size Waste Application Cropland Erosion Pasture Condition
Alabama X - - X - High
Delaware X&D X X X - Low
Maryland X&D X - X - High
Michigan X&D - X&D X&D - High
Utah X&D - - - - High
Vermont X&D - X&D - - High
Wisconsin X X&D - - X&D High
Florida X&D - - - X&D High
Minnesota X&G X - - X High
Oregon X&D - X&D - - High
Pennsylvania X&G - - - - Low
Virginia X&D - X&D - - Medium
* In many projects this was simply any animal operation without treatment

For watersheds in which eroding cropland contributes significant quantities of sediment-attached phosphorus runoff, the distance to the nearest waterway is also important. Other important criteria include cropping system, soil erodibility, land slope, waste application rate, soil fertility, and current management practice. Cropland receiving excess fertilizer may also be critical, depending on location. When nitrogen loading is a major problem, losses are usually not directly related to soil erodibility or sediment movement, but are more closely related to manure or fertilizer application rate and timing, soil type and texture, and area hydrology.

Critical area criteria should be applied consistently throughout the project watershed. Applying all the criteria to each area ensures not only that major pollutant sources receive priority for treatment, but also that landowners whose farms do not meet the criteria do not feel excluded from the program for non-scientific reasons.

Occasionally, identifying critical pollutant sources is simply a matter of observation. In the Utah RCWP project, a small critical area was defined within a large watershed by observing that animal holding corrals were located directly in or in close proximity to drainage ways and, therefore, obviously constituted high priority for treatment. However, critical pollutant source areas are usually not so obvious. Several RCWP projects developed methods for applying criteria to animal waste problems (Table 1). The Oregon and Vermont projects used rating systems based primarily on manure management practices and distance from a watercourse to prioritize farms for treatment. The Vermont project identified critical areas based on a phosphorus (P) load per farm considered treatable with BMPs.

For watersheds with complex hydrology and many different types of pollutant sources, sophisticated computer models may be needed to accurately identify critical areas. The Florida RCWP project used pre-project monitoring and the Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) model (Knisel, 1980) to identify critical sources of phosphorus. This method identified as critical: all dairy operations in the project area, all fertilized and extensively ditched beef cattle pastures, and all agricultural land within one-quarter mile of major streams, ditches, and channels (Stanley and Gunsalus, 1991). Other RCWP projects (Minnesota, Vermont, Illinois, Wisconsin) used computer models after implementing at least some BMPs to evaluate how well pollutant sources had been targeted, or to adjust critical areas.

Distributed parameter water quality models, such as AGNPS, are generally the most accurate tools for identifying critical areas short of actually monitoring the sources. However, considerable expertise and significant amounts of time and effort are required to assemble the necessary model input and to interpret the output. Often, this initial expense is worthwhile, considering the time and money required to design, cost share, and implement BMPs.

Spatial analysis of the watershed using land use survey and hydrologic data in a geographic information system is often useful as an initial estimate of critical areas and, when readily available, can reduce the number of source areas requiring further evaluation.

Water quality monitoring is useful for identifying subwatersheds, tributaries, or land areas contributing significant amounts of pollutants. The Florida and Nebraska RCWP projects used monitoring to document major sources of sediment and nutrients, which facilitated prioritization of critical subwatersheds. Monitoring to confirm critical areas can be relatively simple, such as collecting grab samples at a few key locations over several months.

Summary

Proper identification, prioritization, and treatment of critical areas will significantly improve the chances of mitigating a water quality impairment in a NPS pollution control watershed project. The Idaho, Florida, and Utah RCWP projects documented 40 to 90% reductions in pollutant concentrations by identifying and treating critical areas based on the methods outlined above.

Key Points for Critical Area Determination

Hydraulic Transport of Pollutants to the Water Resource
Magnitude of the Pollutant Source
Type of Pollutant
Type of Water Reource
Severity and Type of Water Quality Program
Methods for Identifying Critical Areas
Summary

References

Gale, J.A., D.E. Line, D.L. Osmond, S.W. Coffey, J. Spooner, J.A. Arnold, T.J. Hoban, and R.C. Wimberley. 1993. Evaluation of the Experimental Rural Clean Water Program. NCSU Water Quality Group, Biological and Agricultural Engineering Department, North Carolina State University, Raleigh, NC, EPA-841- R-93-005, 559p.

Knisel, W.G., ed. 1980. CREAMS: A field-scale model for chemical, runoff, and erosion from agricultural managements systems. Conservation Research Report 26. Dept. Agric. Science and Education Administration, Washington, D.C. 640p.

Stanley, J.W. and B. Gunsalus. 1991. Taylor Creek-Nubbin Slough Project, Rural Clean Water Program Okeechobee, Florida Ten-year Report 1981-1990. Taylor Creek-Nubbin Slough, Florida RCWP local coordinating committee, Okeechobee, FL. 231p.

Young, R.A., C.A. Onstad, D.D. Bosch, and W.P. Anderson. 1987. AGNPS, Agricultural Non-Point-Source Pollution Model: A Watershed Analysis Tool. Conservation Research Report 35. USDA Agricultural Research Service. Washington, D.C. 77p.

Written by

Daniel E. Line and Jean Spooner

Water Quality Extension Specialists

NCSU Water Quality Group

March 1995


North Carolina
Cooperative Extension Service

NORTH CAROLINA STATE UNIVERSITY
COLLEGE OF AGRICULTURAL & LIFE SCIENCES

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

This fact sheet is one of a series of Rural Clean Water Program Technology Transfer fact sheets prepared by the NCSU Water Quality Group with support from the Extension Service, U.S. Department of Agriculture (Cooperative Agreement No. 93-EXCA-3-0241).

Copies of the fact sheet series may be requested from: Publications, NCSU Water Quality Group, Department of Biological and Agricultural Engineering, Box 7637, North Carolina State University, Raleigh, NC 27695-7637, Email: wq_puborder@ncsu.edu, Fax: 919-515-7448.