Biocolloid and solute tracer transport in gravel aquifers - a groundwater protection perspective

Migration conditions in a gravel aquifer of the upper Rhone River valley were studied using particle and solutes as contaminant surrogates. Transport rates were 130 to 480 m/d over distances to 22 in, up to 40 times faster than predicted using conventional flow/effective porosity parameters. In one well, a 1-m vertical pathway heterogeneity dominated the 12-m aquifer saturated thickness. Biocolloids were consistently detected earlier than solutes due in part to their significantly lower detection limits and possibly to preferential particle advection. Biocolloid detection occurred 3- to 7-times earlier than time to solute breakthrough peaks, those values commonly relied on when calculating reference velocity parameters. Relative colloid recovery was typically 1.5 to 4 percent and in one case was 72 % of the solute illustrating relatively low biocolloid attenuation in river gravel macropores. Transport direction was up to 90 degrees off those determined from head-derived measurements. Results suggest that reliable groundwater protection strategy in heterogeneous gravel aquifers may improve when field-verified with migration characterization using multiple tracer types.