Introduction
Nonpoint source (NPS) pollution is the scattered discharge of natural and manmade pollutants into the natural environment. NPS pollution results from widely dispersed runoff over large areas as well as percolation of water into land surfaces. As water flows across land or percolates through soils, it collects salts, metals, soil, nutrients, organic material, or chemicals along the way. These pollutants accumulate as water travels to lakes, streams, wetlands, ponds or groundwater aquifers. When any of these substances accumulate and are deposited into water bodies in amounts greater than naturally occurring levels, they may adversely affect water quality. Negative impacts include:
- higher water temperatures
- higher levels of total dissolved solids
- increases in the numbers of pathogenic microorganisms
- lower levels of dissolved oxygen as a result of algae die off
- nutrient enrichment which can lead to unwanted algae growth
- pH changes
- turbidity or reduced water clarity
These pollutants can result in higher water quality treatment costs, impaired fish and wildlife habitat, poor water quality for livestock and irrigation and reduced suitability of water for swimming and consumption. Water quality experts suggest that agricultural activity is a leading contributor to NPS water pollution.
This fact sheet reviews the types of NPS pollutants that are attributed most often to agricultural activities and specific water impairments included in the 303(d) list. NPS programs and Best Management Practices are also discussed as strategies for controlling NPS pollutants.
NPS Pollutants and Nevada Agriculture The types of NPS pollutants associated with agriculture in Nevada include nutrients, sediment, turbidity, temperature, pathogens, pesticides and salinity. In Nevada, excessive nutrients are the most frequent cause of NPS pollution. One or a number of activities that occur in production agriculture may cause NPS pollution from agricultural lands. These include:
- fertilizer use
- irrigation return flows (tail-waters)
- livestock grazing
- pesticide applications
- soil erosion from fields and roads
- types of irrigation practices
- changes made to the river/stream system
- construction activities
According to the most recently published 303(d) list (2002), Nevada reports 90 waters as impaired. Table 1 summarizes these results and indicates the types of pollutants affecting Nevada's impaired waters and number of water bodies in Nevada impaired. Those impairments that appear in bold-face text are typically associated with agricultural activities.1
Table 1. Impairments affecting Nevada's waters and number of water bodies impaired
| Impairment |
Impairment Number of waters impaired |
| Total Phosphorous |
48 |
| Iron |
40 |
| Turbidity |
26 |
| Temperature |
25 |
| TSS (total suspended solids) |
19 |
| pH |
16 |
| TDS (total dissolved solids) |
15 |
| Mercury |
9 |
| Boron |
8 |
| Arsenic |
4 |
| Selenium |
4 |
| Molybdenum |
3 |
| Copper |
2 |
| Dissolved Oxygen |
2 |
| Nitrate |
1 |
| E Coli |
1 |
| Cyanide |
1 |
| Cadmium |
1 |
The key nutrient associated with agriculture is phosphorous. The second most frequently occurring NPS pollutant related to agriculture in Nevada is excessive sediment (turbidity, TSS, temperature).
Sediment pollution occurs when mineral and organic solid materials erode and are moved away by precipitation, irrigation, air, wind, and gravity. Excessive grazing and trampling of vegetation can increase erosion and runoff that carries NPS pollutants. It can also lead to higher water temperatures due to the decrease in shade provided by some vegetation and due also to wider, shallower stream flows.
Problems with salinity (TSS and TDS) are common in Nevada due partly to its reliance on irrigated agriculture and partly to the composition of soils throughout the state. All irrigation water contains some natural amount of dissolved salts. Salts are also carried to streams from irrigation return flows. The amount of salts increases in waters downstream when irrigation water is diverted and returned repeatedly along a river.
Comparatively, pathogen NPS pollution (E coli) is a minor problem in Nevada. Pathogen problems result from livestock wastes entering water bodies either directly or through agricultural runoff. This problem can exist when livestock grazing occurs in high, concentrated numbers over small areas for lengthy time periods or when livestock graze on or directly beside streams and other water bodies.
Finally, pesticide contamination of surface and ground waters is possible anytime pesticides are applied or stored on agricultural lands. Farmers and ranches must be constantly vigilant in the use of pesticides, fungicides and related chemicals needed for agricultural production.
NPS Programs and Best Management Practices
EPA implements NPS programs in all 50 states designed to help improve water quality through enhancing public awareness, improving irrigation and livestock grazing practices, and improving erosion control measures. Specifically, a state mandated NPS program is required to:
- assess NPS pollution
- establish efforts to control NPS pollution
- administer a federal grant program to assist with implementation of Best Management Practices (BMPs) to reduce NPS source pollution
NPS programs encourage implementation of approved BMPs to control the amounts of NPS pollutants that enter water bodies. NPS programs receive federal grant funds to assist in implementing these programs as required by section 319 of the Clean Water Act (1987). In Nevada, in addition to emphasizing stakeholder education and voluntary use of BMPs to reduce NPS pollution on agricultural lands, NDEP emphasizes watershed planning and coordination efforts to voluntarily reduce NPS.
Best Management Practices (BMPs) are those practices that are determined by NDEP to be the most effective means of preventing or reducing the amount of NPS pollutant in order to achieve water quality standards. There are guidelines for selecting a BMP based upon the pollution generating activity. For agricultural enterprises, nonpoint source pollution can occur from activities related to changes in the hydrologic system in addition to cultural practices that may influence agricultural runoff. A BMP should be tailored to a particular site and specifically address the identified NPS problem.2 It must be cost effective and should include a complete description of the identified NPS as well as the movement of surface and ground waters, soil types, climate, geology topography and habitat. Ideally, a BMP should be professionally designed.
What is a TMDL?
TMDL is an abbreviation for “Total Maximum Daily Load” or the maximum amount of a pollutant(s), measured in pounds per day, which can be discharged into a water body without violating an established water quality standard. TMDLs were originally developed to resolve point source problems. For streams that receive discharges from a variety of industrial and municipal sources, for example, each state measures the pollutant loads from each source in a particular watershed. Reductions in loads from each source are calculated in order to ensure that water quality standards are attained. The limits on pollutants are established and permits issued to point sources of pollution to meet the TMDL of allowable pollutants.
Although TMDLs have been used primarily to identify point sources of pollution, the concept has been extended to include waters impacted by NPS source pollutants or combinations of NPS and point source problems. Therefore, the TMDL concept currently includes watershed assessment of both NPS and point source pollutants. EPA currently focuses on watershed planning to guide NPS TMDLs. Watershed plans should establish goals for reducing percentages of pollutant loads over a period of time through the voluntary use of BMPs. EPA expects states to develop, by the year 2010, TMDLs for all water bodies that are not in compliance with established water quality standards.
Conclusions
NPS pollution is the scattered discharge of natural and manmade pollutants into the natural environment. NPS pollutants most commonly associated with agriculture in Nevada are related to excessive nutrients, sediment and salinity. Pathogens and pesticides are also potential NPS concerns.
NPS programs encourage implementation of approved BMPs to control the amounts of NPS pollutants that enter water bodies. NPS programs receive federal grant funds to assist in implementing these programs. In Nevada, in addition to emphasizing stakeholder education and voluntary use of BMPs to reduce NPS pollution on agricultural lands, NDEP emphasizes watershed planning and coordination efforts to voluntarily reduce NPS and to guide development of TMDLs that target control of NPS pollution. EPA expects states to develop, by the year 2010, TMDLs for all water bodies that are not in compliance with established water quality standards.
References
- Donaldson, S. & M. Hefner. 2004. Nonpoint Source Water Pollution. FS-04-42. University of Nevada, Reno, Extension
- Nevada Division of Environmental Protection. 2002. Nevada’s 303(d) List. Nevada Department of Conservation and Natural Resources. Carson City, Nevada.
- Singletary, L. & J. Davison. 2003. Farm and Rangeland Water Quality Management: A Field Guide for Nevada’s Agricultural Producers.
