Effects of low impact development design on redox dynamics in the vadose zone and urban groundwater

Abstract

Low impact development (LID) best management practices (BMPs) collect, infiltrate, treat urban runoff, and increase recharge to aquifers. Therefore, understanding the redox conditions beneath LID BMPs is essential from a groundwater management perspective. In this study, the influence of design features of a LID BMP infiltration trench on the dynamics of dissolved oxygen (DO) in infiltrating stormwater was investigated. The magnitude and duration of fluctuations in DO concentrations in the LID infiltration trench were found to be influenced by hydraulic conditions in the gravel with an inverse relation between the duration of % saturation in the gravel and DO concentrations. DO reached anoxic levels within hours to days, which strongly indicates that microbial respiration activities are a limiting factor of DO in the gravel. However, temperature of stormwater was not found to exhibit a major influence on redox dynamics in the infiltration trench. The estimated O2 reduction rate for the infiltration trench was found to be 0.003 mg/L minute, which is 2 to 5 orders of magnitude higher than the O2 reduction rates for groundwater aquifers estimated in previous studies. Higher rates of O2 reduction in the infiltration trench are a function of the more oxic flow system and of the higher source of organic matter from stormwater input. The estimated O2 reduction rate in this study strongly indicates denitrification processes are promoted within the gravel. The observed low oxic and anoxic conditions triggered by infiltration of stromwater in the LID infiltration trench are expected to carry on downward through the gravel and probably into the sub-soils beneath and eventually into groundwater. Altering the design of LID BMPs to manipulate redox processes can be an important tool to minimize groundwater contamination.