Dennis W. Westcot
California Regional Water Quality Control Board
Sacramento, California, USA
The management goal of agricultural drainage is to maintain a salt balance in the crop root zone in arid areas and proper soil water balance in humid areas (ASCE, 1990; Smedema, 1990). Drainage water from different locations and/or facilities will have different quality characteristics. Poor quality water should be separated from good quality water. If drainage water is unsuitable for re-use, it should be disposed of in a sink of lower quality water. Highly saline subsurface drainage water from arid areas, re-used for irrigation, could affect the growth of salt sensitive crops. In humid areas, most subsurface drainage water has the potential to be reused. There are several factors to consider when determining the constraints for the management of surface or subsurface agricultural drainage water. The amount and quality of drainage water managed, changes in the rate of flow, and chemical concentrations need to be determined. There are valid references available to provide information on how to conduct field investigations to estimate the volume of drainage water that must be conveyed from the drained area (Luthin, 1957; ILRI, 1994).
Pesticides
Toxic trace elements
Nutrients
Sediment
Bacteria
Temperature
Salinity and major ions
Sulphurous compounds
Water quality is relative and is defined as the characteristic of a water that influences its suitability for a specific use. Quality is defined in terms of physical, chemical and biological characteristics. Drainage water is no different from any other water supply and is always usable for some purpose within certain quality ranges. Beyond these limits, drainage water must be disposed of in a manner that safeguards the usability or quality of the receiving water for present established and potential uses.
Surface and subsurface drainage water from irrigated agriculture is normally degraded compared with the quality of the original water supply. Drainage water that flows over or through the soil will pick up a variety of dissolved and suspended substances including salts, organic compounds and soil particles. Management for safe re-use and disposal requires an understanding of the characteristics of the drainage water, and a matching of those characteristics to the environmental protection needs of the re-use or disposal area. The discussion in this section focuses on the characteristics of drainage water that make it a potential environmental contaminant.
Both surface and subsurface drainage effluent contains substances that are potential pollutants. These pollutants may be:
FIGURE 2 Hierarchical complexity of drainage water management as related to water quality problems (Rickert, 1 993)
i. purposely introduced into irrigation water;
ii. mobilized by the practice of irrigation and/or drainage; or
iii. concentrated as a result of ET.
Each of these three processes requires a different management approach to mitigate its impact. For example, in the first instance, the pollutant source could be removed or limited. By contrast, the concentration of dissolved solids by ET is a natural process that results from consumptive water use and is unavoidable. One or more of these processes may be occurring, thus increasing the complexity of assessing the problem and developing solutions to a given water quality problem. Figure 2 illustrates the differences in the complexity of assessing water quality problems that may be associated with drainage water management.
Numerous types of pesticides may occur in drainage water. This makes it difficult to assess their potential impacts on water quality. Most pesticides are synthetic organic compounds and there are documented instances where organic pesticides in irrigation runoff have caused downstream water quality problems. Recent studies in both the San Joaquin and Imperial valleys of California have shown that surface runoff carries pesticides that cause toxicity problems in aquatic organisms in surface streams (Foe and Connor, 1991; Connor et al., 1993; Di Giorgio et al., 1995). These pesticide problems are the result of farming practices, not the design or functioning of the drainage system. Drainage discharges from irrigation may present elevated concentrations of pesticides, but only for a relatively short period of time. The best solution is improved irrigation water and pesticide management. The drainage system should not be considered a mechanism for controlling pesticide runoff. Those designing and building a surface water drainage re-use or disposal system need to consider whether a pesticide runoff problem is likely to occur, the type of pesticides expected, and the mitigation steps needed to avoid downstream pollution.
High pesticide concentrations in subsurface drainage water are less probable because of the filtering action of the soil. Few recent surveys of subsurface drainage water have been conducted, but groundwater surveys in California show few pesticide detections (California Department of Pesticide Regulation, 1994). Because of infrequent detections, pesticides in subsurface drainage water should not be considered a priority for intensive evaluation unless general water quality surveys show detected levels of concern. However, data summarized by MacKenzie and Viets (1974) show that pesticides have been found in subsurface drainage systems. In the event that pesticides are found, the source is probably a farming practice which can be altered to prevent the continued deep leaching of the pesticide. The presence of a pesticide in subsurface drainage water indicates that the pesticide is in the shallow groundwater and will continue to be discharged with the subsurface drainage for a long period.
Inorganic trace elements are different from synthetic organic compounds (pesticides) in that they are commonly present at low levels in nature and there is already a natural level of tolerance. There is, however, a fine division between natural tolerance and toxicity. It is therefore essential to have good information on the concentration of trace elements in the drainage water in order to develop safe re-use and disposal methods.
High concentrations of inorganic trace elements in irrigated soils and groundwater pose a threat to the environment if they are mobilized by irrigation and drainage practices. They can be collected in the drainage water and then discharged at relatively high concentrations into the environment or at low concentrations and bioaccumulated in the food chain. These trace elements pose a threat to agriculture, wildlife, drinking water and human health.
The trace elements of most importance are those designated as priority pollutants by the USEPA and documented as pollutants associated with irrigated agriculture. Deverel and Fujii (1990) divided these priority pollutants into four categories (Table 2) and reviewed the factors influencing their occurrence and mobility in irrigated soils and groundwater. Recent surveys of subsurface drainage water in the San Joaquin, Sacramento and Imperial valleys of California, have shown that trace element concentrations can be elevated and appear to be strongly associated with the geologic setting of the irrigated area (Deverel et al., 1984; Westcot et al., 1989 and 1993). Deverel and Fujii (1990) also concluded that high concentrations of trace elements in soils and groundwater in arid irrigated areas can occur concomitantly with high soil and groundwater salinity and be affected by the same processes that affect the soil and groundwater salinity. They concluded that high trace element concentrations may also occur independently of salinity such as in humid area soils. Therefore, trace elements should be a priority for evaluation in all drainage projects.
TABLE 2 Geochemical behaviour of trace elements (Deverel and Fujii, 1990)
|
Alkali and alkali earth metals |
Transition metals |
Non-metals |
Heavy metals |
|
Barium |
Chromium |
Arsenic |
Cadmium |
|
Lithium |
Molybdenum |
Boron |
Copper |
|
|
Vanadium |
Selenium |
Lead |
|
|
|
|
Mercury |
|
|
|
|
Nickel |
|
|
|
|
Zinc |
There are no data to show that trace element levels are higher in the surface drainage flow than in the applied water before it moves across a field. Increases in trace element concentrations in surface runoff are generally not expected. Therefore, they are not considered a priority for evaluation.
It is important to be able to predict the potential occurrence of high trace element concentrations in subsurface drainage water. Bowen (1979) and Shacklette and Boerngen (1984) developed range and geometric mean values in soils for the priority pollutant trace elements (Table 3). Comparison of actual soil data with these background levels would give an initial screening as to the potential for trace element leaching. This evaluation should be conducted in conjunction with shallow groundwater sampling for the same potential elements of concern.
TABLE 3 Typical normal acceptable non-harmful trace element concentrations in soils of the western United States (Bowen, 1979; Shacklette and Boerngen, 1984)
|
Element |
mg/kg |
|
|
Baseline (geometric mean) |
Baseline (range) |
|
|
Arsenic |
5.5 |
< 0.01 - 97 |
|
Barium |
580 |
70 - 5 000 |
|
Boron |
23 |
< 20 - 300 |
|
Cadmium |
ND |
ND |
|
Chromium |
41 |
3.0 - 2 000 |
|
Copper |
21 |
2 - 300 |
|
Lead |
17 |
< 10 - 700 |
|
Mercury |
0.046 |
< 0.01 - 4.6 |
|
Molybdenum |
0.85 |
< 3 - 7 |
|
Nickel |
15 |
< 5 - 700 |
|
Selenium |
0.23 |
< 0.10-4.3 |
|
Strontium |
200 |
10- 3 000 |
|
Uranium |
2.5 |
0.68-7.9 |
|
Vanadium |
70 |
7-500 |
|
Zinc |
55 |
1 0 - 2 1 00 |
ND = no data
The two major nutrients in drainage water are N and P. Both contribute to eutrophication of surface waters.
Nitrogen can be in either the organic form (ammonium) or the inorganic form (nitrate). The predominant form in surface drainage is organic N. This comes from the organic matter being washed off the field and should be the focus of any water quality evaluation for surface drainage. This is a persistent problem in higher rainfall areas but is not usually an issue in arid areas. Ammonia is adsorbed on clay particles due to their positive charge. It can also volatilize. Recent sampling of surface runoff from irrigated areas in the San Joaquin Valley of California showed almost no nitrate in the surface runoff. This was attributed to little nitrate N (NO3-N) fertilizer being applied to the soil surface.
Nitrate is the dominant form of N in subsurface drainage water. Nitrate N should be the focus of any evaluation of water quality from a subsurface drainage system. High nitrate concentrations in subsurface drainage can originate from a number of sources: geologic deposits, natural organic matter decomposition and deep percolation of nitrate resulting from fertilizer applications. Nitrate contamination of subsurface drainage water has been documented by Madramootoo et al. (1992). Mineral N, in the form of nitrate or nitrite, is transported in a dissolved form. The proportion of these various forms in drainage water depends on the predominant form of drainage. Nitrite is the most dangerous form of N. It is in general a transient form of N, present in small quantities.
Agricultural drainage water also contains phosphate in both organic and inorganic forms. Most of the phosphate in surface drainage is in the organic form. Little phosphate has been found in subsurface drainage water because of its strong adsorption in arid zone soils (Johnston et al., 1965; MacKenzie and Viets, 1974) and in humid regions (Madramootoo et al., 1992).
While rainfall induced erosion is a significant source of sediment, irrigated agriculture may also cause erosion directly through application of irrigation water, or indirectly through sub-optimal land management. Sediment contained in surface runoff from agricultural lands may carry P and certain pesticides to surface waters where they may contaminate the food chain or affect other beneficial uses of water. Excess sedimentation can also degrade the stream environment, diminish the health and diversity of fish and wildlife habitats, limit recreational uses and add to the costs of flood management and drinking water treatment.
Because subsurface drainage water is primarily groundwater, it is not expected to carry significant amounts of sediment. No data are readily available on sediment loads in subsurface drainage water, but sediment from subsurface drains is not normally expected to be a significant issue. However, from time to time a subsurface drain line will be poorly constructed, or a drain line will fail after construction and a significant amount of sediment will collect in the drains. This will then either plug the drains so they will not function or be discharged at the drain outlet. Moreover, in certain soils, drains with inadequate or improperly installed envelopes can accumulate fine sands and silts and these sediments will eventually either plug the drains or be discharged with the drainage water. In any event, the normally relatively sediment free water from subsurface drains might erode the banks of unlined surface drains and increase the sediment load of the drainage water.
Erosion is a natural geologic process. However, although sediment cannot be eliminated, its movement can be controlled to reasonable levels.
Bacteria are a potential pollutant where surface return flows come from land that has received applications of human or animal waste. Bacterial pollution may also originate from wetland discharges. The focus of a bacteriological contamination assessment is on the measurement of coliform and faecal coliform levels in the water. A normal irrigated farming operation would not be expected to produce adverse bacteriological levels in surface drainage water. The presence of coliform or faecal coliform would be an indication that other types of wastewater, such as municipal, industrial or animal waste, may have entered the surface drainage system.
Soil is a biological filter. Therefore, it is not expected that micro-organisms will move through the soil from surface water to a subsurface drainage system. There is no record of coliform or faecal coliform levels being measured in a subsurface drainage system, nor are they expected to be a problem.
Elevated temperatures occur where irrigated fields or wetlands are warmed by the sun, and tailwater from these areas is then discharged into a stream, causing a rise in the stream temperature. This problem is often aggravated where diversions for irrigation and wetland management also reduce the total stream flow. Elevated temperatures have a direct impact on stream aquatic life especially in certain cold water streams or those with anadromous fisheries. Temperature surveys should be an essential component of any surface water monitoring if elevated water temperatures are expected. Power plants sometimes affect the temperature of downstream waters.
In contrast to surface drainage, subsurface drainage is mainly groundwater and has a relatively constant water temperature. The temperature of subsurface drainage waters can be easily predicted as they approach average seasonal or annual temperatures. Measurements in the San Joaquin Valley of California, show that subsurface drainage discharges average 17°C, with a variability of less than 2°C (Chilcott et al., 1988). These cool discharge temperatures would not be expected to have an impact on warm water fisheries as they quickly come into equilibrium with the ambient air temperature.
The practice of irrigation results in consumptive uses of water, leaving behind salts concentrated in a smaller volume of water. During irrigation, two types of drainage may occur: surface drainage and subsurface drainage. Reeve et al. (1955) showed that the salt content of water changes little in flowing over soil even where there is a visible salt crust. Studies of tailwater throughout the San Joaquin Valley of California have also shown that the chemical composition of the surface drainage is essentially that of the supply water.
Salts are a major water quality factor in choosing disposal options for subsurface drainage in arid irrigated areas. Salinity can restrict the urban or agricultural re-use of drainage water, as it is the most significant long-term water quality concern for managing irrigated agriculture in arid zones. Salinity has not been noted as a serious concern with subsurface drainage waters from humid areas. This is generally due to the higher rainfall, higher dilution capacity in surface waters and lower initial salt content in the soil.
In arid irrigated areas, irrigation practices mobilize naturally occurring salt in the soil and concentrate those salts already present in the supply water. The salts captured by a subsurface drainage system are often highly concentrated with the major cations being Na+, Ca2+, Mg2+ and, to a lesser extent, K+. The major anions are Cl-, SO42-, HCO3-, NO3- and CO32-. Most subsurface drainage waters are NaSO4 and NaCl dominated. These are common non-toxic elements that only become problematic when concentrated in the soil. However, some MgSO4 waters have been observed. The predominant cations and anions influence the re-use and disposal methods chosen.
Subsurface drainage water from arid areas always has a higher salinity than the supply water, a higher proportion of Na and Cl, an increased hardness and a higher sodium adsorption ratio (SAR). The higher salinity and higher levels of specific ions often reflect the characteristics of the soil through which the irrigation water has percolated. This in turn is influenced by the shallow groundwater quality, by the ionic composition of the irrigation water, and by the irrigation efficiency. Salt species are also influenced by a number of interdependent, multi-phase chemical interactions. There are a number of models available to predict solute migration in soils (Jurinak and Suarez, 1990). A full salinity appraisal is an essential component of any subsurface drainage water re-use or disposal scheme.
Water quality problems are often encountered when sulphaquepts or acid sulphate soils are drained. Such soils are mostly found in tropical river deltas. Improved drainage of sulphaquepts results in an increased discharge of acid water. High quantities of sulphuric acid may be released into receiving streams. There is also a reduced pH which can affect aquatic life. Furthermore, iron and aluminium in these soils can be mobilized and may cause human health problems if the receiving water bodies are used for domestic drinking water.
Domestic and drinking water
Industrial supply
Agricultural supply
Aquatic life
Recreation
The re-use and disposal of agricultural drainage water must be environmentally sustainable. This means allowing other uses to be made of the water resources that will not be adversely affected by the drainage project. Understanding the water quality needs of the various downstream uses can assist in developing mitigation methods. The primary needs to be considered are drinking water, industrial supply water, agricultural supply water, recreational uses and aquatic life.
Surface and subsurface drainage water discharges can degrade the potability of drinking water and increase the cost of testing, treatment and delivery. An additional concern is human contact with the water during bathing, washing clothes and recreational use. Domestic use and drinking water safety is a primary consideration for most nations and for the general public. Drinking water supplies can be affected by sediment, nutrients, micro-organisms, total salts, sulphate, chloride and pesticide levels. Sediment increases water treatment costs. High nutrient levels, especially nitrate above 45 mg/litre as nitrate, are a potential public health problem. High nutrient levels may also cause algae problems which can lead to a disagreeable taste and the formation of THM precursors. Bacteria levels indicate a high potential for disease problems and may increase water treatment costs. High total dissolved solids (TDS) can cause taste problems. Recommended levels of salinity are less than 0.9 dS/m with a maximum suggested at 2.0 dS/m. Sulphate and chloride levels are recommended between 250 and 500 mg/litre (Marshack, 1993). The higher salinity of subsurface drainage water may also increase treatment costs due to the increased water hardness.
Pesticide levels in drainage water should be checked if a discharge is planned into a known drinking water supply. Trace elements may also cause toxicity. Recommended trace element levels in drinking water are shown in Table 4.
TABLE 4 Summary of measured concentrations of trace elements in shallow groundwater from the San Joaquin Valley, California (Deverel et at,, 1984; Chilcott et al., 1988 and 1990), and established US water quality criteria for drinking water, aquatic life protection (Marshack, 1 993) and irrigation (Ayers and Westcot, 1985)
|
Constituent |
Measured |
Criteria |
|||
|
m g/litre |
|||||
|
Median |
Maximum |
Drinking water standard |
Irrigation guideline limits |
Aquatic life protection limits |
|
|
Arsenic |
44 |
1 400 |
50 |
100 |
46 |
|
Boron |
3 100 |
1 20 000 |
- |
- |
- |
|
Cadmium |
< 1 |
2 |
10 |
10 |
0.5 |
|
Chromium |
10 |
170 |
50 |
100 |
11 |
|
Copper |
2 |
23 |
1 000 |
200 |
5.4 |
|
Iron |
50 |
7 400 |
300 |
500 |
- |
|
Lead |
<1 |
17 |
50 |
5 000 |
0.9 |
|
Lithium |
90 |
430 |
- |
2 500 |
- |
|
Manganese |
30 |
2 500 |
50 |
200 |
- |
|
Mercury |
< 0.1 |
1.6 |
2 |
- |
0.1 |
|
Molybdenum |
640 |
6 600 |
- |
10 |
- |
|
Nickel |
47 |
2 815 |
600 |
10 |
650 |
|
Selenium |
6 |
3 800 |
10 |
20 |
35 |
|
Uranium |
170 |
3 100 |
- |
- |
- |
|
Vanadium |
61 |
945 |
- |
100 |
- |
|
Zinc |
11 |
620 |
- |
2 000 |
47 |
The median value for TDS in Deverel et al. (1 984) was 2 350 mg/litre. Values shown in italics for the median and maximum values are from Chilcott et al. (1988 and 1990); the other values are from Deverel et al. (1984).
Water supplies for industrial processing are affected primarily by high salinity levels. The major concern is increased hardness as this could cause scaling or precipitation and increase costs for treatment prior to use. The discharge of drainage water into a known industrial supply should focus on the salinity and hardness limits of the water.
Secondary effects of drainage waters on industrial usage might be related to nutrients, sediment, micro-organisms and pesticides. These may restrict the type of use the water is put to, or may increase treatment costs prior to use.
Agricultural water users are primarily affected by salinity or the individual salts in the water supply. Total salinity can cause crop yield reductions and increased costs for salt management. Specific salts such as boron, chloride and sodium also cause plant toxicity problems. In addition, sodium can cause soil permeability problems. As subsurface drainage water itself is commonly higher in total salts, sodium, chloride and boron, agricultural water users can be affected by its discharge into usable water supplies. There are several published guidelines on agricultural use of higher salinity water (Ayers and Westcot, 1985; ASCE, 1990; and Rhoades et al., 1992).
Trace elements are of concern for irrigation or animal drinking water use. Suggested guideline values for irrigation use are given in Table 4. Animal drinking water guidelines are given in Ayers and Westcot (1985) along with a discussion of the guideline values for irrigation. In most instances, trace element levels acceptable to irrigated agriculture may be lower than those required for other uses, such as aquatic life, so agricultural use may not be the most limiting. The exceptions are molybdenum and selenium which can accumulate in crops and cause toxicity to the consumer of the crop (Page et al., 1990).
Public values for the protection of aquatic and wetland wildlife are concerned with the survival, health and diversity of fish, wildlife and aquatic plants. If discharged incorrectly, pollutants in subsurface drainage may be acutely toxic or bioaccumulate. They may also damage plant and animal health in the long term, or impair the reproductive ability of aquatic species.
The pollutants of primary concern in surface drainage water are pesticides, sediment and temperature. Elevated temperatures can cause damage to cold water fisheries. Sediment levels can cause instream damage to habitat or restrict light penetration and aquatic food development. With surface drainage, the pollutant of greatest concern is pesticides. However, there are few criteria available on allowable pesticide levels in water. It is therefore suggested that pesticide surveys and analysis concentrate on aquatic toxicity. This can be defined by the USEPA's three species test (USEPA, 1994).
The pollutants of primary concern in subsurface drainage are trace elements. Table 4 lists recommended water quality limits for trace elements for aquatic life protection. In most instances, trace element levels in natural waters are very low (Westcot et al., 1990b). Recent experience shows that trace element levels in subsurface drainage may be elevated (Tables 3 and 4) and often exceed those found in seawater or inland salt lakes (Westcot et al., 1990a).
In the San Joaquin Valley of California, impacts on aquatic life due to drainage water discharges have occurred. In this area, there are six potentially toxic trace elements known to exist in concentrations sufficient to cause impacts on aquatic life: As, B, Hg, Mo, Se and U. It is recommended that all drainage areas be surveyed for these six trace elements due to their potential harmful impacts on aquatic life. Mercury and Se are of particular concern for aquatic life because of their bioaccumulative nature, even at very low concentrations in the water. Boron is known to be harmful to agricultural crops and similar impacts on aquatic plants would be expected. Arsenic is a known human carcinogen at very low levels and could have a similar impact on aquatic life and wildlife, though no data are available (Deverel and Fujii, 1990). Plants tolerate high levels of Se and Mo, but both elements are known to concentrate in forage crops and can have an impact on the livestock or wildlife that consume the crop. Little or no data are available for U. Trace elements such as Cu, Fe, Mn and Zn have little solubility in saline and alkaline soils and would not be expected to be present under such conditions (Page et al., 1990).
Recreation in water is becoming an important part of many economies. The discharge of subsurface drainage water could affect the aesthetic or recreational value of a water body and impair that component of the economy. The impairment, in addition to the effects on aquatic life as discussed above, would be to restrict human contact with the drainage water due to high pesticide, sediment or bacteria levels. These impacts would generally come from surface drainage systems. A secondary concern with recreation and associated with subsurface drainage water is the discharge of nutrients that may cause eutrophication and stimulate aquatic plant growth. This leads to a loss of aesthetic values in recreational waters.
