Richard E. Green

STATISTICAL AND GRAPHICAL METHODS FOR EVALUATING SOLUTE TRANSPORT MODELS: OVERVIEW AND APPLICATION

Abstract Mathematical modeling is the major tool to predict the mobility and the persistence of pollutants to and within groundwater systems. Several comprehensive institutional models have been developed in recent years for this purpose. However, evaluation procedures are not well established for models of saturated-unsaturated soil-water flow and chemical transport. This paper consists of three parts: (1) an overview of various aspects of mathematical modeling focused upon solute transport models; (2) an introduction to statistical criteria and graphical displays that can be useful for model evaluation; and (3) an example of model evaluation for a mathematical model of pesticide leaching. The model testing example uses observed and predicted atrazine concentration profiles from a small catchment in Georgia. The model tested is the EPA pesticide root zone model (PRZM).

Uncertainty in a pesticide leaching assessment for Hawaii

Abstract In this paper we report the predictive uncertainty associated with using the retardation factor ( RF ) as an index to evaluate pesticide leaching in Hawaii soils when uncertainty exists within the soil and chemical data used to excite the RF model. Our analysis takes two separate but ultimately related tacks. First, we assess the uncertainty caused by extrapolating soil properties between taxonomic categories. Second, we characterize the amount of uncertainty for calculated RF values due to data uncertainty by using first-order uncertainty analysis. Our results indicate that the RF index should only be used with soil information from the lowest taxonomic category and that even then considerable uncertainty will exist in the predicted RF values used to screen and rank chemicals.

Impact of uncertainty in soil, climatic, and chemical information in a pesticide leaching assessment.

A simple mobility index, when combined with a geographic information system, can be used to generate rating maps which indicate qualitatively the potential for various organic chemicals to leach to groundwater. In this paper we investigate the magnitude of uncertainty associated with pesticide mobility estimates as a result of data uncertainties. Our example is for the Pearl Harbor Basin, Oahu, Hawaii. The two pesticides included in our analysis are atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and diuron [3-(3,4-dichlorophenyul)-1,1-dimethylarea]. The mobility index used here is known as the Attenuation Factor (AF); it requires soil, hydrogeologic, climatic and chemical information as input data. We employ first-order uncertainty analysis to characterize the uncertainty in estimates of AF resulting from uncertainties in the various input data. Soils in the Pearl Harbor Basin are delineated at the order taxonomic category for this study. Our results show that there can be a significant amount of uncertainty in estimates of pesticide mobility for the Pearl Harbor Basin. This information needs to be considered if future decisions concerning chemical regulation are to be based on estimates of pesticide mobility determined from simple indices.

Evaluation of a pesticide mobility index: impact of recharge variation and soil profile heterogeneity

The attenuation factor (AF) is subjected to a preliminary assessment of model error by comparing the simple index method with the more conceptually rigorous pesticide root zone model (PRZM). The model error associated with the AF assumptions of a homogeneous soil profile and time-invariant recharge rates are tested. Both the AF and PRZM models are used to rank the relative mobility of ten organic chemicals for conditions in Hawaii. The results presented here indicate that AF compares well to PRZM.

Transformation and adsorption of Fenamiphos, f. sulfoxide and f. sulfone in molokai soil and simulated movement with irrigation

Abstract The ban of commonly used soil fumigants, DBCP and EDB, for control of nematodes in pineapple fields has prompted investigations into a non-fumigant nematicide, fenamiphos (Nemacur®). The transformation and adsorption in soil of fenamiphos and its transformation products, f. sulfoxide and f. sulfone were studied in the laboratory. Fenamiphos adsorption on soil exceeded that of f. sulfoxide and f. sulfone. F. sulfoxide, however, was the most persistent. A one-dimensional simulation model was used to assess the impact of transformation and adsorption on the mobility and distribution of fenamiphos and f. sulfoxide in soil. Simulated results showed that fenamiphos stayed in the topsoil and transformed rapidly to f. sulfoxide. Because of the persistence and mobility of f. sulfoxide, this metabolite leached rapidly and significant amounts remained in the soil. This suggests that for times exceeding three weeks, f. sulfoxide may be the dominant compound providing nematode control in drip-irrigated pineapple.

Modeling of radon transport in unsaturated soil

This study applies a recently developed model, LEACHV, to simulate transport of radon through unsaturated soil and compares calculated soil radon activities against field-measured values. For volatile and gas phase transport, LEACHV is modified from LEACHP, a pesticide version of LEACHM, a well-documented one-dimensional model for water and chemical movement through unsaturated soil. LEACHV adds consideration of air temperature changes and air flow driven by barometric pressure change to the other soil variables currently used in LEACHP. It applies diurnal barometric pressure and air temperature changes to reflect more accurately the typical field conditions. Sensitivity analysis and simulated results have clearly demonstrated the relative importance of barometric pressure change, rainfall events, changes in water content, gas advection, and radon source term in radon transport process. Comparisons among simulated results illustrated that the importance of barometric pressure change and its pumping phenomenon produces both fluctuation in soil gas radon activities and an elevation of the long-term average radon activity in the shallow soil. Barometric pressure pumping was found to produce an effect on radon activity in shallow soils of an equal magnitude to the distributed source parameter. Comparison between measured and simulated soil radon activities showed that LEACHV can provide realistic estimates of radon activity concentration in the soil profile.

Criteria for Evaluating Pesticide Leaching Models

Mathematical models which permit query of “what if” scenarios are timely tools for assessing pesticide leaching potentials. Well-defined evaluation procedures for these important models have not, however, been firmly established. In this paper criteria for model evaluation are presented. Three examples of model testing are given. The models tested include a simple index of mobility and a deterministic-empirical simulation algorithm. The model testing examples make use of pesticide leaching data from Georgia and Hawaii.

Reducing variance in soil organic carbon estimates: soil classification and geostatistical approaches

Abstract Concern for groundwater purity is increasing. Computer modeling of processes that can contribute to groundwater contamination is growing, yet the supporting data base is meager and the available data were usually not collected for such uses. Often one is left to estimate soil properties from data on analogous soils. Soil organic carbon, a major input in pesticide leaching assessments, can be extrapolated to unsampled sites by two approaches: by analogous reasoning using Soil Taxonomy and by regionalized varible methods (geostatistics). The 23×32 km Pearl Harbor recharge area was used to compare these two methods of estimating soil organic carbon in unsampled areas. Soil organic carbon varied from 0.013 to about 0.060 g C/g soil. The general variance for 53 samples in the area was 1.04×10 −4 and, according to gestatistical estimates from semivariograms, could be reduced to 1.08×10 −5 at a sampling distance of 1.2 km. Based on a transect sampling study within the area, increases in sampling frequency from 4600 to 7.36 million samples would not further reduce estimated variance and thus would not be advisable. Extrapolation on the basis of Soil Taxonomy, i.e. to similar soil series, and geostatistical extrapolation provided similar estimates of uncertainty in soil organic carbon in unsampled areas. Data from the Pearl Harbor recharge area suggest that extrapolation by Soil Taxonomy may underestimate regional varibility due to sampling assumptions. Extrapolation by geostatistics ignores some important qualitative information. A method of combining these two approaches is needed. Our results emphasize the effect of the purpose and assumptions of a sampling plan on the data obtained. Large scale modeling of soil properties of a region, typical of many geographical information systems (GIS) analyses, requires a representative sample of the variation in the region. A three-phase sampling plan that includes a qualitative phase as well as a quantitative phase may ensure that the data obtained are appropriate.

Wellhead treatment costs for groundwater contaminated with pesticides: A preliminary analysis for pineapple in Hawaii

In Hawaii, trace concentrations of pesticides used in the production of pineapple were found in the groundwater supplies of Mililani Town in the Pearl Harbor Basin on the island of Oahu. Groundwater serves as the major source of drinking water and residents pay for wellhead treatment of the contaminated water, via their monthly water bill. The agricultural chemical users within the Pearl Harbor Basin do not include these wellhead treatment costs in their production costs. The agricultural industry benefits from using pesticides but does not pay the entire societal cost of using these chemicals. In this study we evaluate the specific financial cost of wellhead treatment, and not the economic value of groundwater. While wellhead treatment costs could conceivably be shared by several parties, this study focuses on the financial impact of the pineapple industry alone. This study factors annual wellhead treatment costs into annual pineapple production costs to measure the effect on annual financial return from pineapple production. Wellhead treatment costs are calculated from the existing granulated activated carbon (GAC) water treatment facility for Millilani Wells I and II. Pineapple production costs are estimated from previous cost of production studies. The inclusion of wellhead treatment costs produces different production-cost results, depending on the scale of analysis. At the local scale, the Mililani wellhead treatment costs can be factored into the production costs of the pineapple fields, which were probably responsible for contamination of the Mililani Wells, without causing a deficit in economic return. At the larger regional scale, however, the return from all of the pineapple grown in the Pearl Harbor Basin can not sustain the cost of wellhead treatmentfor the entire water supply of the basin. Recommendations point to the prevention of groundwater contamination as more cost-effective measure than wellhead treatment.

Transport of a degradable substance and its metabolites under drip irrigation

Abstract A two-dimensional model of water flow and transport of noninteractive solute in soils under drip irrigation, is extended to incorporate degradation pathway of a degradable parent substance and its metabolites. Adsorption of the parent material and its products to soil is also considered. The model yields two-dimensional distributions of water content and of the substance and each of its metabolites, and has a potential to be used for selected degradable substances, soil properties and management alternatives of practical interest to farmers and environmentalists. A sample calculation was accomplished for a single infiltration-redistribution cycle, with a parent substance applied in water as a line source during infiltration into a particular soil with a given set of hydraulic properties and coefficient of adsorption and degradation.