Statistical techniques for screening experimental or literature chemical databases for compounds exhibiting potential environmental activity are becoming increasingly utilized in environmental analysis as pragmatic and economical complementary tools to enhance or augment costly traditional analytical procedures. Utilizing the predictive modeling approach, it is often argued, implicitly permits an unlimited number of chemicals to be screened for specific behavioral or physicochemical characteristics in a variety of environmental and biological matrices, consequentially conserving the financial resources for exhaustive testing, yet providing a methodology that helps to insure that questionable compounds are more thoroughly tested. Moreover, such techniques provide a database of exhaustive test results from which investigators and regulators can extract relevant information for further research or decision-making.

To assess the efficiency of statistical modeling methods for predicting chemical processes in the environment, a one-year exploratory study utilizing Quantitative Structure-Activity Relationship (QSAR) methodology to obtain linear model equations for estimating the rates of chemical hydrolysis of several organophosphorus (OP) pesticides in natural river waters has been conducted. This modeling effort specifically considers the effects of chemical structure on reactivity and utilizes connectivity parameters from graph theory as quantitative structural descriptors. Derived model equations were examined to establish whether quantitative correlations between fundamental molecular characteristics and observed hydrolytic properties were possible. Inconclusive results for a training set of six OP pesticides indicate that there are inherent weaknesses in molecular connectivity theory when applied to complex reaction parameters that require further exploration. The inherent complexity of most chemical reaction mechanisms and the indistinct influence of both adjoining and distant atoms in the molecular environment makes it difficult for a single descriptor, even one as widely successful as connectivity indices, to adequately account for definitive structural characteristics of molecules. It is apparent from results of this study that molecular connectivity indices alone are often not discriminating enough descriptors for procuring comprehensive structure-property relationships beyond a rather restricted range of structural variation, at least when characterizing chemical reaction parameters.

Agricultural drainage from Delta islands is known to be a significant contributor of DOC (Dissolved Organic Carbon) that forms TI-IMs (Trihalomethanes) when drinking water is chlorinated. The current agricultural practices create seasonal wet-dry cycles in the fields so that salinity, sodicity, temperature and moisture content of soils are varied. This study was carried out to understand the influences of the current agricultural practices on the production of DOC and THM from surface (oxidized) and subsurface (reduced) peat soil of the Sacramento-San Joaquin Delta. Both abiotic and biotic factors are examined independently in order to identify the major DOC and THM precursors production processes. Abiotic factors examined with successive batch soil solution extraction were salinity (EC from °to 4 dS/m) and sodicity (SAR from °to co) that affect the coagulation and dispersion of soil organic matter (SOM). Biotic factors examined with S-week batch incubation were temperature (lO, 20, 30 oC) and moisture (30, 70, 200 % moisture) that affect the rate of microbial decay of SOM, producing CO2 and DOC.

In the abiotic experiment, the results showed that the increase of the soil-water salinity decreases the amount of Doe and decreases its aromaticity. A decrease of SAR also decreases the amount of DOC and decreases its aromaticity. The results of the abiotic experiment showed that the salt accumulation of the summer irrigation is not the major production of DOC; instead, the salt accumulation may reduce the DOC leaching from the peat soil, if we consider the salt effects alone.

In the biotic experiment" the 8-week incubation experiments showed that temperature, moisture content and wet-dry cycles affect the microbial activities in soils, but only the flooded and the wet-dry cycle incubations increase the DOC concentration in the oxidized peat soil. However, the extracted DOC from the incubated peat soils showed lower STHMFP although there was increases in the concentration and SUV254' In summary, the current agricultural practices alter the soil salinity and create the wet-dry and flooded conditions in the fields. The summer irrigation increases the soil salinity. However, the increase of salinity decreases the DOC productions but increases the STHMFP. On the other hand, the wet-dry cycle in the summer and flooded conditiOlf in the winter did produce the DOC, but the STHMFP of DOC produced in these conditions decreased.

Sulfide toxicity to rice plants has been randomly observed in isolated sites in rice fields and experimental plots in the Sacramento Valley. Plants suffering from sulfide toxicity show signs of retarded growth and reduced yields, including the characteristic blackened roots and, in the most severe cases, death. Because the environmental conditions causing sulfide toxicity are not clear, we carried out a greenhouse pot test. The treatments included straw and sulfate additions to a rice soil to induce sulfide production. Our results contribute to an improved definition of conditions leading to sulfide production, toxicity and impact to rice plants.

Salt management has been a challenge to westside farmers since the rapid expansion of irrigated agriculture in the 1900 s. The soils in this area are naturally salt-affected having formed from marine sedimentary rocks rich in sea salts rendering the shallow groundwater, and drainage return flows discharging into the lower reaches of the San Joaquin River, saline. Salinity problems are affected by the imported water supply from Delta where the Sacramento and San Joaquin Rivers combine. Water quality objectives on salinity and boron have been in place for decades to protect beneficial uses of the river. However it was the selenium-induced avian toxicity that occurred in the evaporation ponds of Kesterson Reservoir (the terminal reservoir of a planned but not completed San Joaquin Basin Master Drain) that changed public attitudes about agricultural drainage and initiated a steady stream of environmental legislation directed at reducing non-point source pollution of the River. Annual and monthly selenium load restrictions and salinity and boron Total Maximum Daily Loads (TMDLs) are the most recent of these policy initiatives. Failure by both State and Federal water agencies to construct a Master Drain facility serving mostly west-side irrigated agriculture has constrained these agencies to consider only In-Valley solutions to ongoing drainage problems. For the Westlands subarea, which has no surface irrigation drainage outlet to the San Joaquin River, innovative drainage reuse systems such as the Integrated Farm Drainage Management (IFDM) offer short- to medium-term solutions while more permanent remedies to salt disposal are being investigated. Real-time salinity management, which requires improved coordination of east-side reservoir releases and west-side drainage, offers some relief to Grasslands Basin farmers and wetland managers - allowing greater salinity loading to the River than under a strict TMDL. However, current regulation drives a policy that results in a moratorium on all drainage return flows.

Evaporation ponds are utilized to dispose of saline agricultural drainage waters where there are no surface drainage outlets to the San Joaquin River. Presently, 22 ponds in the San Joaquin Valley's west side are receiving about 32,000 acre-feet per year of tile drainage effluents and about 0.8 million tons per year of salt are being deposited. This project investigates the incorporation of potentially toxic trace elements (selenium, boron, arsenic, molybdenum) into evaporite minerals forming by desiccation of drainage waters.

Over 50 samples of salt deposits (evaporites) obtained from seven ponds show that the predominant minerals are either halite (Ne Cl), thenardite (Na2S04) or mirabilite (Na2S04 • 10H20), reflecting the dominant dissolved mineral salts in the drain water. The highest selenium concentrations in evaporites were found in Peck pond, ranging from 3 to 33 mg/kg (ppm) with a mean value of 12 mg/kg for six samples. Evaporite samples from all other ponds contained a maximum selenium level of 1.8 mg/kg and mean value of 0.22 mg/kg or less. The values are well below the 100 mg/kg threshold level of selenium for wastes to be considered hazardous. The concentrations of boron in evaporites ranged from a low of 16 in Barbizon pond to as high as 975 mg/kg in Lost Hills pond. The mean concentration of boron in evaporites from seven ponds ranged from 73 to 690 mg/kg. There are no hazardous criterion for boron threshold level in waste solids.

Laboratory studies indicate that association of boron with halite or mirabilite are influenced by both the nature of the salt crystal surface and the chemical species of boron. For example, boron association occurs mainly by surface adsorption on halite and by both surface adsorption and internal absorption (by diffusion into the crystal lattice) for mirabilite,