Assessment of Nutrient Use and Nitrate Contamination in Central Oahu, Hawaii

Abstract

Groundwater contamination by agricultural fertilization is a widely recognized problem in the USA and worldwide. Here in Hawaii, agricultural fields have posed a threat to the invaluable basal aquifer. The concentration of nitrate-N in the Pearl Harbor aquifer on Oahu was below 2.3 mg/L in the 1950’s and 1960’s, but has increased in some wells in the Kunia area to a maximum of 7.6 mg/L in 1992 to 1994. The objective of this study is to assess nitrogen use in the agricultural lands and nitrate contamination in the Pearl Harbor aquifer. Nitrate distribution and migration in the subsurface are influenced by many mechanisms. Nitrogen fertilizer undergoes many N transformations and interactions with the soil and the plant after applications. In this study, a field data collection was done and analysis of the samples was completed to understand the leaching process of nitrate in the root zone of three different cropped fields. A detailed discussion is presented to address various factors that control the transport process. Field measurements were evaluated statistically to judge the sampling plan in relation to spatial variations. The study uses an uncertainty index in the analysis, which is the density of samples required for a sample mean to fall within a defined limit of accuracy. To simplify the process of estimating leaching rates to the aquifer, a simple lumped parameter model (LPM) was developed. The model, which is analytical in nature, uses a limited set of input data. Average leaching rates can be estimated in response to agricultural practices, N transformations, and other processes. The model is tested against two detailed numerical models with great success. The model will be made available for interested parties for use in assessing the potential threat to aquifers. Various types of uncertainties affect our ability to predict nitrate fate accurately. Five major sources of uncertainty were identified in this study: (1) uncertainty due to spatial variation, (2) uncertainty related to the accuracy of the input data, (3) uncertainty due to simplifications in the development of the models, (4) uncertainty due to modeling parameters, and (5) uncertainty due to the complexity of the unsaturated zone in Hawaii. The impact of these uncertainties on the simulations of nitrate leaching is evaluated and a sensitivity analysis was done to quantify the uncertainty due to the modeling parameters in a leaching model. The result showed that hydraulic properties, plant uptake, and dispersivity are very important in achieving reliable simulation or prediction. It is recommended that hydraulic conductivity and water retention relationships be measured under field conditions before the use of the model. A historical record of specific plant growth and nutrient assimilation by the plant can provide reasonable data for modeling the plant uptake process. Dispersivity can be obtained by calibration of the model in a specific soil. Although the sensitivity analysis suggests that uncertainty in nitrate adsorption exert a considerable effect on nitrate transport results, anion adsorption is not a principal factor that affects nitrate leaching in the root zone. The relatively high content of organic matter in the root zone may favor a net negative surface charge on soil colloids, which contributes to the leachability of negatively charged nitrate. In contrast however, retardation of nitrate plays a dominant role in nitrate leaching below the root zone in central Oahu. Only a small fraction of the nitrate leaching reaches the groundwater. Although different explanations have been proposed for the sorption process in the thick unsaturated zone in central Oahu, a retardation factor should be considered for interpreting the results of the outflow from the root zone. Nitrate contamination in the Pearl Harbor aquifer was assessed. Quasi three-dimensional models were used to simulate water flow and nitrate transport in the aquifer. The models were calibrated by using available records and used for predictions based on assumed scenarios for water and land use. Future predictions show the likelihood of a nitrate contamination problem. Assumed conservative nitrate concentrations at the water table and a very mild increase in water use resulted in concentrations that exceed the MCL for nitrate of 10 ppm. Nitrate concentration below the agricultural lands is likely to increase with planned land use changes that may require additional fertilizer use. The possibility of changes in the physical or chemical ability of the unsaturated deep formations to store nitrate may also lead to increased leaching rates. A need exists for a better management of fertilizer use, a process that should be possible without sacrificing agricultural productivity. The approaches developed in this study can help in such an endeavor.