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1 California California is a major agricultural state where nearly 25 percent of the pesticides used each year in the United States are applied. California's groundwater is a vast natural resource of vital importance to the economic growth and development of the state. The current under- standing of the extent or seriousness of agricultural chemicals in groundwater is extremely limited and dis- jointed because of the incomplete data base currently available. Nearly everyone interviewed agreed that additional sampling and monitoring of water quality trends is needed to establish the extent of pesticide contamination of groundwater. An engineer with many years of experience On June 21, 22, 25, and 26, 1984, Golden visited with various individuals in California either directly familiar with groundwater problem-= caused by agricultural chemicals or working in disciplines with the potential to reduce future problems. The institutions r epresented by the individuals interviewed include the University of California, Davis and Riverside campuses--sources of out- standing basic agricultural research for many years; the California Department of Food and Agriculture (CDFA)--the state agency responsible for pesticide registration, use permit processing, and enforcement; the California Water Resources Control Board (WR$B)--the state agency respon- sible for the protection of groundwater quality; and the Water Quality Control Board (WQCB), Central Valley Region--one of nine regional boards that work directly with growers and make water quality decisions subject to appellate review by the WRCB. 14
15 said that the problem of groundwater contamination by organic chemicals that are not regulated as groundwater pollutants is Virgin territory and that regulatory agencies are groping in the dark as to how to proceed. Complicating the picture is the fact that little or no baseline and trend water quality data exist for the approximately 460 groundwater basins in California. THE STATUS OF EFFORTS TO MONITOR GROUNDWATER FOR RESIDUES OF AGRICULTURAL PESTICIDES Despite the limited data base, some specific problems are well established. Agencies in California have sampled approximately 8,000 wells for the nematocide DBCP. me pesticide was found in more than 2,000 wells and is known to have contaminated gro~andwater in an area encompassing 7,000 square miles of the San Joaquin Valley. DBCP was used in California from the late 195Gs until its registration was canceled in August 1977. It was used mainly in areas with sandy soils, including the east side of the San Joaquin Valley, but the actual acreage to which it was applied is not known because accurate records are not available for the entire period of use. However , typical annual application rates ranged from 20 to 80 lbs per acre, with total usage in California from 1960 to 1977 estimated by the CDFA to be approximately 3 million lbs per year. Unfortunately, the type of comprehensive, coordinated monitoring effort undertaken to assess the extent of DBCP contamination has been rare. Past Monitoring Efforts - Historically, monitoring for pesticides has been a fragmented and uncoordinated series of special studies of limited duration and scope. Many of the sampling programs were conducted under time and budgetary con- straints in response to a specific contamination incident and thus lacked adequate sampling protocol development. Past monitoring efforts have been severely limited in two ways. First, in most of the California groundwater monitoring program, samples were collected from existing wells because of the high cost of installing monitoring wells. Second, samples were typically analyzed for a limited range of pesticides that were chosen on the basis
16 of their use in a given area, their potential for leach- ing, or problems caused by their use in another basin. Other agricultural chemicals may have been present in the groundwater but were not detected in the chemical- specific analysis used to quantify the pesticides of concern. Multiresidue analytic techniques to better assess water quality in areas susceptible to contamina- tion from agricultural chemicals need to be developed. But despite these significant gaps, more than 50 different pesticides have been found in the groundwater of 23 California counties. The most extensive monitoring effort was the 1979-1982 DBCP sampling program, which revealed that the pesticide was present in approximately one out of four wells tested. Most of the samples with detectable DBCP levels had concentrations below 100 ppb. A high correlation between the presence of DBCP and elevated nitrate levels in groundwater suggests that DBCP was transported to the groundwater via percolating irrigation water. Other pesticides widely monitored in groundwater include 1,2-dibromoethane (EDB), 1,2-dichloropropaneJ 1,3-dichloropropene (D-D), simazine, atrazine, and carbofuran. me confirmed presence of these pesticides in California's groundwater is much less extensive than that of DBCP. A 1983 study by the WRCB found that the water in 67 wells of 266 sampled (25 percent) contained 1,2-dichloropropane, a manufacturing by-product present in the soil fumigant D-D. A 1982 monitoring program by the CDFA examined samples from 217 well sites chosen to tap shallow aquifers in four groundwater basins of agr icultural areas. Four pesticides were tested for: 1. DBCP--detected in water from 27 wells (12 percent); concentration: 0.1-10.5 ppb. 2. The herbicide simazine--detected in water from five wells (2 percent); concentration: 0.5-3.S ppb. (Originally, the herbicide sampled for was atrazine, but none was detected; instead, si-=zine was found.) 3. The nematocide EDB -detected in water from two wells ( 1 percent); concentration : 0 . 1-0 . 2 ppb . 4. The insecticide carbofuran--detected in water from one well (0.5 percent); concentration: 0.5 ppb. The CDFA sampling program was not established to characterize the basin-wide groundwater quality with reference to the chosen pesticides. Additional or more extensive groundwater contamination caused by the target
17 pesticides could exist in the four basins, since only one well in a 36- or 72-square-mile area was sampled. By the same token, a positive finding is an inadequate basis for projecting that such a large area is widely contaminated with a given pesticide. Current Monitoring Efforts Current monitoring efforts in California remain frag- mented and lacking in scope and depth. Funding levels for monitoring programs within the various agencies have increased only slightly, remained constant, or even declined. Despite increasing concern about contamination of groundwater resources, the research budgets for development of baseline and trend water quality data and for special investigative projects have not risen signi- f~cantly. In fact, the WRCB budget for development of baseline water quality data has been cut over the last 5 years from $400,000 to approximately S240, 000. This level of funding has constrained the state WRCB, which is the only agency currently developing baseline and trend data on groundwater basins. Of the 461 groundwater basins in California, 160 are heavily pumped for water consumption. ~- ~ or these 160 basins, 24 have been desig- nated Priority I based on a comparison of beneficial uses, population, present water quality, and stress on water quality. _ _ Of the 24 Priority I basins, the W~CB selected 4 for its initial monitoring. Monitoring efforts began in 1978 with a sampling site concentration of approximately one well per square mile. Twelve toxic organic chemicals (including the pesticides EDB, DBCP, aldicarb, and D-D), toxic metals, nitrates, and some minerals were selected for monitoring. Althougn the program was established to collect baseline and trend water quality data, routine sampling of some wells has been halted for years due to funding restrictions. In fiscal year 1984, the approximate budget for this program was 3240,000, which causes obvious constraints when the estimated analytic cost alone is S250-S300 per sample. Consequently, baseline and trend data back to 1978 are currently being gathered on only eight Priority I ground- water basins in California. Nevertheless, this remains the only continuous groundwater monitoring program in California attempting to evaluate background and trend data for an entire basin. NO other western state, and _ _
18 few states nationwide, have groundwater monitoring programs of the scope and quality of California's. A broad special investigation initiated in 1984 involves a one-shot sampling effort of all wells serving as sources of public water supplies. The samples are tested for the presence of organic chemicals. This investigation was authorized by California Assembly Bill 1803, Passed in September 1983, as one response to concerns about toxic chemicals in drinking water sparked by the rice herbicide controversy in the Sacramento area (see subsection Best Management Practices for Pesticide Use). ~ _ The California Department of Health Services (CDHS) is responsible for implementing the statewide program and for establishing which industrial and agricultural chemicals will be subject to analysis relative to land use in the vicinity of a particular public supply well. If contaminants are found in the initial screening, follow-up sampling will be conducted. The sampling and analytic costs will be borne by the public water supply systems. Whatever the technical shortcomings of this ad hOC program, it is generally perceived as a constructive response to the growing level of public alarm regarding groundwater contamination. Another current monitoring effort by the WRCB is the shot spot. program, or 205(j) Project, funded through the federal Clean Water Act. The intent of this program is to develop sampling methods and analytic techniques that can be used by the WRCB and regional WQCBS in the event of a contamination problem. It will consist of highly focused studies that investigate specific contaminants in worst-case situations. At present the hot spot program has $400,000 to initiate a 2- to 3-year study of - specific chemicals at sites to be chosen based on the greatest likelihood of preexisting groundwater contamina- tion. These sites and chemicals have yet to be determined, although an industrial chemical, such as trichloroethylene, and an agricultural chemical will be chosen. Limited field testing of predictive models is planned. Finally, sampling mandated by the Safe Drinking Water Act (SDWA) is carried out by public health agencies on wells serving public water supply systems. Six pesti- cides ar e tested for: the insecticides endrin, lindane, methoxychlor, and toxaphene; and the herbicides 2,4-D and 2,4,5-TP (Silvex). However, many uses of these pesti- cides, especially endrin, lindane, toxaphene, and 2,4,5- TP, have been curtailed by regulatory actions or limited _ _ . . ~
19 by the availability of more efficacious, modern com- pounds. In addition, the chlorinated hydrocarbon insecticides are not very mobile in the soil/water environment. Although 2,4-D and 2,4,5-TP are more mobile, they are relatively nonpersistent (soil half- lives of less than 20 days) and thus are rarely observed in groundwater. Consequently, sampling conducted under the SDWA does not represent an important data base with regard to contemporary or future groundwater problems caused by pesticides. The most recent and comprehensive review of pesticides in California groundwateY is presented in Table 1-1, which shows the number of positive detections and the maximum concentrations. This table includes numerous detections associated with point sources, but the data do not distinguish sources in individual cases. CRITICAL PROBLEMS AND NEEDS Nemo tocides One class of agricultural chemicals , nematocides, poses a particularly high risk of groundwater contamina- tzon. Nematocides are designed to be mobile in the soil/ water environment in order to protect the root zone, where pest problems occur. Furthermore, most severe nematode problems occur in sandy, porous soils with low water-holding capacity, which increases the likelihood of leaching to groundwater. m e nematocide DBCP has caused the most extensive groundwater contamination from an agricultural chemical documented to date in the United States. Other nematocides, such as EDB, D-D, and aldicarb, have also leached to groundwater. The problem of nematocides reaching groundwater presents a difficult management problem with no obvious resolution. A Hematologist at the University of California, Riverside, believes there is a need to rethink control of nematodes with an eye toward protect- ing groundwater resources. Staff at the CDFA voiced a similar concern. Interdisciplinary Research A neither of those interviewed felt that the land grant university system needs a broader, more interdisciplinary
20 TABLE 1-1 Pesticides in California Groundwater Number of Positive Detects ( total number of wells Maximum EPA-Suggested sampled if information Concentration Analytic Pesticide availablea) (ppb) Methodb Aldicarb 17 (106) 47 c Aldrin 21 17.8 Basegran 1 20 Benzaldehyde 1 2 Chlordane 4 22 608 CIPC 1 8 Dacthal 4 35 DBCP 2,000+ (8,000+) 1,240d 502 1,2-l)ichloropropane 68 (266) 1,200 601 Deloev 5 2 5 DDD 3 3 DOE 15 5 DOT 10 2 0 DEF 1 1.7 Diazinon 13 9 Diclone 1 2.7 Dieldr in 6 5 Dimethoate 24 190 Dinoseb (DABS) 14 740 615 Diphenamid 1 6,000 Disyston 7 6 DtdOC 5 3 5 Dursban 3 go EDEN `5 . 140 502 Endosulfan 17 100 Endr in 1 4 0 60 8 Ethion 5 30 Ethylenethiourea 1 7 Furadan 2 5 632 ace 2 0 .0 5 Heptachlor 3 0.3 608 ECelthane (Dicofol) 3 1.99 Lindane 32 46 608 Malathion 5 23 Methylenechlor ide 4 7 Omite 2 92 Ordram 3 6.3 Paraoxon 1 6 Parathion 3 4 PQIB 2 0 .3 PCP 38 44,000,000 Oe 604 Phorate 2 20 Phthalates 4 10 Sideline 9 (217) 0.53 619 Sevin 3 80 Treflan 1 0.9 TOP 6 980 Toxaphene 5 123 608 Zytron 4 30 2,4D 11 4.0 615 2,4,5T 3 1.4 2,4,5-TP 3 1.0 615 NOTE: The California W~CB found some errors in the compilation of data by Ramlit Associates, Inc., used to produce this table of their report. The W~CB rechecked the data to verify the detections and concentrations listed, and this table incorporates those corrections of January 1985. The Ramlit study includes not only detections associated with field use of pesticides, but also detections associated with manufacturing facilities, mixing and loading facilities, and so on.
21 TABLE 1-1 (Continued) Information on the total number of wells sampled is not available f rom the California WRCB for most of the pesticides listed bNumbers given refer to approved EPA analytic method for pesticides selected for national monitoring survey. The analytic technique associated with each number is as follows: 502--purge and trap gas chromatography (GC); 601--purge and trap GC; 604--solvent extraction GC: 608--solvent extraction GC; 615-derivation -=C; 619--solvent extraction GC; 632--solvent extraction high-pressure liquid chrome tography ( HP~C ) . CEPA is developing an HPLC-based analytic procedure for the determination of ald icart>. dFrom well near DBCP manufacturing plant in Lathrop, California. ePresumably f rom a point source--for example, where wood is treated. SOURCES: Ramlit Associates, Inc., for the California WICK, 1983, Groundwater Contamination bv Pesticides: A California Assessment; California Depart~nen~c of Food and Agriculture, 1983, Pesticide Movement to Groundwater, Vol. 1: Survev of Ground Water Basins for DBCP EDB Sideline and Carbofuran Materials submitted to the , . . . FIFRA Scientific Advisory Panel by Union Carbide Agricultural Products Co., Inc., June 12, 1984 . research effort to address the groundwater issue. Some felt that while considerable money has been available historically for research related to efficacy of pesti- cides, comparatively little money was available to study their environmental fate and movement. It was the view of these persons that the potential for groundwater con- tamination from pesticides and the agricultural manage- ment strategies to mitigate such problems have not been given sufficient attention by schools of agriculture. Land grant universities appear to be logical centers to address this problem. Data Base and the Need for a Multiresidue Analytic Screen A lack of water quality data impedes efforts to deter- mine the severity of groundwater contamination caused by agricultural chemicals. Often, samples from wells have not been analyzed for pesticides. In other cases the monitoring was limited in scope with regard to areal extent or analytic sensitivity to pesticides of concern or was done so poorly that it is meaningless. Not enough water quality data are available to assert with confi- dence that groundwater contamination by pesticides is a broad problem. The data base needs to be expanded, but
22 such efforts are hindered by the high costs of establish- ing monitoring programs and performing analyses. A multiresidue analytic screening technique is critically needed to drive down the cost of sampling for a wide range of pesticides. Irrigation Practices Improved irrigation efficiency can help reduce the potential for deep percolation of irrigation water carrying dissolved pesticides, fertilizers, and salts as solutes. If minimal water percolates through the vadose zone to cause recharge of an aquifer, the direct pathway available for a pesticide to move through the unsaturated zone to the water table is also minimized. However, poorly constructed wells without annular seals in place to prevent surface water or soil water in the first few meters from migrating down the side of the well casing to the groundwater can serve as direct pathways to aquifers. Corroded, damaged, or fully screened casing could cause similar problems. Efficient irrigation scheduling can provide the water needed tn Dreuent water stress to a crops while minimize ing the amount of water percolating beyond the root zone. Two complicating factors in attempts to reduce pesticide leaching via irrigation water are (1) the need to apply excessive water in some regions of the country to leach soluble salts out of the root zone and (2) the need to provide enough irrigation water to meet the requirements of the least permeable areas of a field, consequently providing an excess to the remainder of the field. The excess water can carry some pesticides beyond the root zone, through the vadose zone, to the groundwater. Thus, more efficient irrigation may reduce pesticide contamina- tion of groundwater, but is unlikely to eliminate it. One state water quality official interviewed believed that relatively inexpensive water available to growers in California prompted excessive irrigation and thus contributed to the groundwater problems in the San Joaquin Valley. Higher priced irrigation water most probably would result in more efficient use of the resource and could lead to the substitution of drip irrigation for flood irrigation methods.
23 Dry Wells Another possible cause of groundwater contamination is the channeling of irrigation tailwater (that is, surface runoff from flood irrigation, which can contain significant levels of pesticides and fertilizers) down dry, abandoned wells. This practice can short-circu~t the natural percolation mechanisms that degrade organic chemicals present in the water. In the Central Valley region alone, approximately 5,000 abandoned dry wells could be used for this purpose, out the actual extent to which such wells contribute to groundwater contamination is unknown. Ground and Aerial Applicators Work yards of ground applicators and airstrips used by aerial applicators of pesticides have on occasion been the sites of poor disposal practices of residual pesti- cide solutions and tank rinse waters. Approximately 500 aerial and ground applicator sites exist in the Central Valley Region alone. In some cases, severely contamin- ated soil and water have been severely contaminated. For example, at the Butter County Airport, toxaphene was found in excess of 100,000 ppm in soil, and in Kern County the soil near one airstrip was found to be nearly saturated with DOT (dichlorodipnenyltrichloroethane; a relatively nonmotile, persistent chemical) from the soil surface to the water table, and thus had contaminated groundwater at low levels. Generally, however, the problems at these sites are considered to have been caused by outmoded practices. Current practices rely on tank rinse water recycling or on spraying three tankfuls of rinse water on the fields just treated. But, largely due to funding restrictions, no comprehensive assessment of the applicator sites for potential groundwater contamination has occurred, nor are such sites closely regulated. Delineation of Regulatory Areas The wide range of hydrologic conditions in California results in great variations in the recharge of aquifers by rainfall or irrigation water and thus in the possi- bility that pesticides will contaminate groundwater.
24 Thus, officials and staff interviewed at the CDFA indicated that the selective restr lotion of pesticides in certain areas where the site-specific hydrogeology indicates that groundwater is highly vulnerable to contamination is preferable to statewide bans. One member of the CDFA staff noted, however, that attempts to map boundaries in a complex and varied hydrologic setting like California would impose considerable technical challenges that could severely tax the expertise and resources of state agencies. Also, the implementation of such a strategy would be difficult because the delineation of areas vulnerable to ground- water contamination would certainly not coincide with existing political boundaries, nor be readily accepted by all parties at risk. Public and Institutional Perceptions of California Groundwater Contami net ion Citing the number of public inquiries and requests for agency publications, officials in California water agencies pointed out that the average California citizen is becoming increasingly aware and concerned about the threat posed to groundwater resources by toxic organic chemicals from both agricultural and industrial sources. Usually, however, the public does not differentiate between agricultural and industrial sources of toxic chemicals and perceives DBCP in the San Joaquin Valley, the industrial solvent trichloroethylene (TCE) In the Santa Clara Valley, or toxic wastes inadequately disposed of at the Stringfellow Acid Pits in Southern California as representative of ~ unitary threat to groundwater resources. In other words, the origin, relative toxicity, and pervasiveness of the different contaminants are not separated. The lack of sophistication in public thinking about the general issue of groundwater quality is a consider- able worry to the CDFA as agricultural chemicals continue to appear in analyses of groundwater samples. As public pressure mounts to protect groundwater, the CDFA is concerned that public responses to future agriculture- related problems could result in pressure to ban pesti- cides vital to the interests of California agriculture. The CDFA believes that more intelligent use of pesticides with a high potential for causing groundwater problems is needed and that selective restrictions are the best
25 way to balance growers" needs for effective agricultural chemicals with the need to protect groundwater resources underlying agricultural areas. The state WRCBs and regional WQCBs are charged with the protection of groundwater quality, and are concerned about potential contamination of groundwater by many toxic organic chemicals used in California by industry and agriculture. With regard to agricultural chemicals, the regional WQCBs appear willing to work with growers to develop strategies to mitigate groundwater problems. However, while best management practices can be developed and recommended to growers, the WQCBs have no policing or enforcement mechanism. Management practices that could mitigate potential problems may not be followed by growers; in fact, it is suspected that label recommenda- tions on application rates for pesticides are ignored by some growers. Nevertheless, the actions of rice growers in the Sacramento area--described under the subsection Best Management Practice for Pesticide Use--indicate that a combination of effective techniques and perceived need can lead to the adoption of management solutions. Researchers visited at the University of California expressed varying degrees of concern about contamination caused by agricultural chemicals. Some of the agricul- tural scientists felt the problem was principally restricted to nematocides (DBCP, EDB, aldicarb, D-D, and so on) applied to sandy soils with shallow water tables. They expressed the view that few, if any, additional problems existed. Others felt the problem could poten- tially be much broader and that an integrated research effort cutting across disciplines was critically needed to assess the dimensions of the problem and develop strategies to mitigate future problems. AGRICULTURAL MANAGEMENT STRATEGIES AVAILABLE TO MITIGATE PESTICIDE/GROUNDWATER QUALITY PROBLEMS Irrigation Efficiency The potential and limitations of irrigation efficiency to reduce pesticide leaching have already been mentioned. A flow of irrigation water that is sufficient to satisfy the leaching requirement for western soils, abut that minimizes deep percolation and recharge, could help mitigate pesticide contamination of groundwater. Improved efficiencies could reduce water consumption for
26 irrigation 25-30 percent. And although field irrigation efficiencies for most surface irrigation systems (furrow, border, basin) are probably not more than 60 percent for the western states as a whole, specially designed systems using drip irrigation or laser leveled basins are thought to yield efficiencies of greater than 90 percent. Best Management Practices for Pesticide Use Best management practices (BMPs) to mitigate c ontamr ination of groundwater from the use of pesticides include closely following label instructions, carefully calibrating spray equipment, efficiently scheduling arrogation, optimizing timing of pesticide applications, altering cropping patterns, and properly disposing of tank rinse water or residual pesticide solutions and containers. While everyone interviewed considered these BMPs important, some were skeptical of the effectiveness of promoting them, believing that some growers would likely ignore such recommendations for a variety of reasons, including lack of economic incentive and their determination to provide a stress-free growing environ- ment for their crops. The potential for reducing pesticide contamination problems through combined irrigation and BMPS was recently demonstrated in the resolution of a rice herbi- cide controversy in the Sacramento area. In this case, which involved discharges to surface waters, irrigation water containing the herbicides Bolero (thiobencarb) and Ordram (molinate) was released from fields into the Sacramento River and caused the eventual presence of these pesticides in the drinking water of various communities that use the Sacramento River as a source of potable water. Because of the detection of Bolero in the water--up to 2.8 ppb at the City of Sacramento's water intake--and the bitter taste the chemical imparted to the water, the California rice industry found itself having to defend the use of Bolero and other herbicides from regulatory action. Working quickly, the rice industry, with help from the University of California and the Central Valley Region WQCS, began an educational campaign to encourage growers to adopt management strategies that could reduce the pesticide load released to the Sacramento River. The management strategies included (1) holding irrigation water in the fields up to 8 days after the late spring
27 application of Bolero and Ordram to allow the pesticides in solution more time to degrade fully and (2) irrigating more efficiently with more recycling of water. Those recommendations have been widely accepted and practiced by the rice growers. The alternative was perceived to be the potential loss of the chemicals due to pressures brought to bear by a concerned and upset public. As a result of this program, releases of irrigation water from some drains have dropped dramatically. Daily monitoring of the river water revealed that concentra- tions of Bolero had been reduced 50 percent during late spring, when the concentrations peak. Concentrations are expected to continue to decline as a larger percentage of growers follow the recommendations. This example could serve as a model for future surface and groundwater cases involving agricultural chemicals where adoption of BMPs have the potential to reduce environmental degradation while preserving the avail- ability of a chemical for growers. By slightly altering their practices, the rice growers lessened the environ- mental degradation associated with their operations and most likely avoided the cancellation of use permits for agricultural chemicals believed to be important for efficient production. Integrated Pest Management (IPM) Integrated pest management is the coordinated appLica- tion of a range of pest management techniques designed to manage pests at levels below economic damage thresholds. Careful scouting of pest populations, biological controls, and crop management techniques are the first line of defense against pests. Chemical pesticides are used only as required, and the levels are typically less than levels applied under routine spraying schedules, thus reducing the possibility of groundwater contamination. California has been a leader in the development of IPM techniques. However, experts interviewed varied considerably in their confidence in the availability of IPM for major crops. It appears that IPM has the potential to reduce the total pesticide load and, hence, the residues introduced into the soil/water environment. But additional research is needed to develop and refine IPM strategies through field testing so that extension personnel can make their
28 recommendations with confidence and so that the tech- niques can compete economically with the alternatives. Growers may not make use of such pest control measures, however, until forced by circumstances such as insect resistance to specific agricultural chemicals. Formulation Chances . Formulation changes are a strategy to reduce ground- water contamination. The shift through formulation changes from O-D (Shell) to Telone II (Dow) eliminated a constituent that was contaminating groundwater. The active ingredient in both products is the nematocide 1,3-dichloropropene (1,3-D). me original D-D product marketed by Shell contained 30-35 percent 1,2-dichloro- propane (1,2-D), which is relatively ineffective as a soil nematocide. Although the toxic constituent 1,2-D was discovered in eight counties in the water from shallow and deep wells, the less mobile, less persistent 1,3-D has never been found in groundwater in California. The logical solution was to reduce or, if possible, eliminate 1,2-D from the formulation of the original D-D product. Currently, Telone II contains 92 percent 1,3-D and 8 percent inert ingredients, of which less than 2 percent is 1,2-D. Thus, Telone II remains available but the threat to groundwater is diminished. MODELS TO PREDICT THE TRANSPORT AND ENVIRONMENTAL FATE OF PESTICIDES Pioneering work on simulation models to describe the movement of trace organics in soil has been done in California. (A more detailed description of some models appears in Chapter 2 on New York.) Currently, the bulk of the work on models has been in the laboratory with columns of soil of known texture and structure under controlled conditions. Experimenters are Reasonably confident. about the usefulness of these models in a controlled environment. However, current models offer no precise pesticides field. method of predicting the depth of movement of in soil or the concentration at depth in the The use of simulation models in field situations is in an incipient state. me EPA's Environmental Monitoring Systems Laboratory In Las Vegas is developing
29 a report to assist researchers engaged in field verifica- tion of simulation models. Entitled Guidelines for Field Testing Soil Fate and Transport Models, it is expected to be published in 1986. Problems with using models to simulate the fate and transport of pesticides in the soil/water environment in the field include uncertainties related to soil hetero- geneity, macropores, and microbial activity. The spatial variability of heterogeneous soils repre- sents a considerable problem in the application of laboratory models to field situations. Generally, the models currently available compute the rates of leaching and degradation as a function of tome. Average water percolation rates through the soil and vadose zone are used, but these may not conform to actual field rates. Also, factors such as permeability, organic matter content, texture, structure, hydraulic conductivity, and depth of soil horizons can vary widely in a single field And predictions of the transport and environmental fate of a chemical, based on observed environmental condi- tions, could be quite different within a relatively small area. In addition to soil heterogeneity, other features in the soil/water environment such as macropores could cause sizable errors when simulation models are used to predict pesticide movement in the field. Macropores are structural features in soil that characteristically allow the ready movement of percolating water. Because of macropores and soil heterogeneity, individual ~parcels. of water in the soil do not all travel at the same rate. For most hydrologic purposes, one need only predict or measure the average rate of water transport, not the rate of movement (velocity) of an individual parcel. For assessing the movement of pollutants, however, one must know these individual parcel rates. At present, the models to predict these parcel velocities and the tech- niques to quantify the necessary parameters for a hydro- logic basin, or to sample individual field performance, are still in the early stages of development. Another Sopor ten t process not fully amenable to current simulation models is the variable rate of degra- dation of pesticides by soil-dwelling bacteria and fungi Microbial activity is believed to be a major contributor to the degradation of organic chemicals in soil. Because the concentrations and types of microbes in soil vary considerably from field to field, biodegradation rates vary widely as well. Biodegradation is a complex and not
30 fully understood process that is difficult to incorporate into mathematical models. Perhaps for the reasons just outlined, and because of the particular needs of regulators, the state personnel at the WRCB, WQCB, and CDFA were skeptical of the use of simulation models for predicting the environmental fate of pesticides in the soil/water environment. The consensus was that the models were currently untested and unproved in field situations and, thus, could not be used with confidence outside the laboratory. It is generally believed, however, that simulation models could be of considerable value to regulators and manufacturers attempting to assess the environmental fate and impact of a specific chemical prior to registration and intensive manufacturing once they have been verified in the f ield.
