Development of the slowly released molasses barrier system for controlling nitrate plume in groundwater

Abstract

This study was objected to identify the applicability of the well-type slowly released molasses barrier system (SRM system) as an in situ remedial technique to treat the nitrate-contaminated groundwater by the denitrifying activity of heterotrophic microbes. A SRM material, a solidifying molasses, was made using a molding technique by mixing the liquid phase molasses with paraffin wax, cellulose, and silica sands. This SRM material can continuously release molasses as a carbon source for indigenous heterotrophic denitrifiers over relatively long periods by the diffusion process with decreased release rates when it is placed into groundwater passing through. The developed SRM system could continuously deliver molasses into the groundwater over an extended period of time. Therefore, the SRM system could be an attractive long-term nitrate treatment option, showing nitrate removal efficiencies were estimated to be fairly high as ~85% and ~84% for nitrate of 89 and 142 mg L-1 in the column- and pilot-scale experiments. The removal efficiencies in the field experiment were relatively moderate at ~43% for nitrate of 320 mg L-1. Moderate nitrate removal efficiency in the field might have been caused by the heterogeneity/anisotropy of the aquifer and insufficient molasses dispersion to the denitrifiers. From result of the PCR-DGGE series, nitrite reductase gene fragments were amplified from uncultured isolates in pilot- and field soils, indicating a heterotrophic denitrifying capability in soil is common. Thus, the SRM system can be possibly applied into the nitrate contaminated groundwater with minor consideration for the existence of the denitrifying microbes in soils. Although many constraints related to the heterogeneous nature of aquifers exist, the SRM system can be a useful tool for control of dilute, large, and shallow nitrate contaminated groundwater plume. For achieving on-site remedial goals of the nitrate-contaminated groundwater in the targeted field, changes in the mixing rate of SRM constituents or its volume to prolong effective longevity can be readily modified. Further, the number and shape of the SRM barrier and the array of the SRM rods per a barrier can be changed for attaining remedial goals of the targeted contaminated field.