Using Laboratory Settling Experiments to Characterize the Suspended Load or Movable Matrix Portion of an Urban Aquifer: Potential Relationships to Land Use

Abstract

Urban groundwater has received considerable study over the past several decades due to dissolved contamination and subsequent remediation; however, less attention has been paid to the solid suspended load, settleable, or suspendible and movable (SMS) portion of the aquifer system. This study investigated the fine fraction (<10 μm) of solids present in groundwater wells penetrating a shallow (3–7 m below grade) unconsolidated urban aquifer. The goal of the investigation was to determine concentrations of SMS and predict particle-size distributions via a laboratory settling experiment. Data were collected utilizing visible light attenuation spectroscopic measurements, in conjunction with dynamic imaging particle analysis. Settling data from representative standard wells were fit with exponential functions and used to predict sizes in other wells within the study area. Predictions were checked against particle-size measurements from scanning electron microscope analysis. Groundwater samples were examined from the following land uses: street, sidewalk, surge basin, residential lawn, managed lawn, parking lot, former industrial, and drainage surge ditch. Measured SMS initial concentrations ranged from approximately 6,600 to 1,300 ppm for particles ranging in size from about 7 to 1.5 μm. SMS size distributions can be categorized into bimodal, multimodal, or positively skewed. Some land-use trends were observed, with samples from street sites containing particles of a roughly bimodal or multimodal size distribution, surge basins having a positively skewed distribution, and samples from ditch and managed lawn locations containing an approximately multimodal size distribution.