CHARACTERIZATION OF SUB-MICRON SUSPENDED LOAD IN A SHALLOW AQUIFER: CORRELATION TO URBAN LAND USE

Urban groundwater has received considerable study over the past 30 years due to contamination and subsequent remediation. However, less attention has been paid to the solid suspended load in urban groundwater. This study investigated the fine, near-nano fraction (4.2-0.7 µm) of solids present in groundwater wells penetrating a shallow (3-7m below grade), homogeneous, sandy Midwest aquifer. Two relationships were examined: (1) Whether suspended particle concentration and size distribution are related to current or historical surface land use; and, (2) Whether naturally-occurring suspended fractions can be distinguished from anthropogenic suspended load. Samples were collected from 11 wells and 1 drainage ditch within a 1 km²area. Land use areas consisted of street, lawn, sidewalk, parking lot, former industrial location, storm surge basin on former industrial location, municipal drainage ditch, and surface drainage ditch sites.

Suspended particle concentrations and size distributions were determined using spectrophotometric techniques, dynamic imaging particle analysis, and adapted Navier-Stokes settling calculations. Suspended load chemistry was evaluated by SEM-EDS and PXRD analysis and was utilized as a first approximation to distinguish naturally-occurring material from anthropogenically-derived particles.

Preliminary results show the total suspended load concentrations (SLC) range from ~ 140 to 4900 ppm. The highest SLC concentration was observed in the groundwater (GW) beneath a residential lawn. GW under streets, sidewalks and surge basins had intermediate SLC values (1500-2550 ppm), and GW associated with a former industrial site and parking lot had consistently lower SLC (280-1030 ppm). Composition analysis of the ≤ 0.76 µm fractions from different locations indicated that silicates of quartz, feldspar and clay are common natural suspended matter throughout the aquifer. The most conspicuous anthropogenic material is graphite/graphitized carbon black, distinguishing the street, sidewalk and surge-basin sites from other land uses. Results suggest that current land-use practices may have greater influence on GW SLC, while historical activities more heavily influence particulate chemistry.