Hydraulic conductivity and porosity of soil/saprolite cores in Dead Run watershed, Baltimore County, MD USA

Soil/saprolite cores were collected at eight hilltop locations in the headwater area of Dead Run watershed in Baltimore County, MD, and evaluated for hydraulic conductivity and total porosity. The lithology consists of Mount Washington Amphibolite (mafic rock) transitioning to the Potomac Group of the Fall Zone. Cores were collected using an AMS 10.2 cm-diameter steel auger bucket, in increments of about 15 cm, to depth of refusal. The length of the soil cores ranged from 1.4 m to 6.5 m. Each ~1200 cm3 sample was evaluated in the laboratory for hydraulic conductivity (K) using a UMBC/CUERE-designed falling-head permeameter and the Hvorslev equation. For each of 180 samples, 6 measurements were taken and corrected to a water temperature of 20˚C. Average K values ranged from 10-9 to 10-5 m/sec, with a geometric mean of 4.9x10-7 m/sec and an lnK variance of 2.7. The pattern of hydraulic conductivity varied with depth. K decreased by ~ 1-2 orders of magnitude to a depth of about 0.5 m. As the soil transitioned to saprolite, K values then increased by 1-2 orders of magnitude to depths of 2 - 2.5 m. For longer cores, a decrease in K values of 1-2 orders of magnitude was observed below 2.5 m, followed further zonal order-of-magnitude increases/decreases over 1-2 m depths. Results illustrate pronounced zonal heterogeneity of hydraulic conductivity for this system. As observed at other field sites, hydraulic conductivity values were log-normally distributed. Total porosity was determined on the same samples using a Teros 12 soil moisture sensor. Four measurements were taken. Average total porosity ranged from 0.31 to 0.53, with a mean of 0.45 and variance of 0.0016. Total porosity generally decreased with depth (0.1 – 1m), followed by an increase and then leveling off to a relatively constant value with depth. The zonal heterogeneity of total porosity was most prominent from 1-2 m, as opposed to hydraulic conductivity, which exhibited zonal heterogeneity with greater depths for longer core samples.