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.

于美国马里兰州巴尔的摩县Dead Run流域源头区域的八个山顶位置采集了土壤/风化壳岩心，对其进行了水力传导率和总孔隙率的评估。岩石学组成由华盛顿山角闪岩（基性岩）过渡至fall zone的Potomac组。岩心采用直径为AMS 10.2厘米的钢制钻头斗采集，深度以约15厘米的增量进行，直至达到拒绝深度。土壤岩心的长度介于1.4米至6.5米之间。每个约1200立方厘米的样本在实验室中通过UMBC/CUERE设计的降头渗透计和Hvorslev方程式评估其水力传导率（K）。对于180个样本中的每一个，进行了6次测量，并校正至20°C的水温。平均K值介于10^-9至10^-5米/秒，几何平均值为4.9x10^-7米/秒，lnK的方差为2.7。水力传导率的分布模式随深度变化。K值在约0.5米的深度下降了约1-2个数量级。随着土壤过渡到风化壳，K值随后在2-2.5米的深度增加了1-2个数量级。对于较长的岩心，观察到在2.5米以下K值下降了1-2个数量级，随后在1-2米的深度范围内进一步出现区域性的数量级增加/减少。结果展示了该系统中水力传导率明显的区域异质性。正如在其他现场观测点所见，水力传导率值呈对数正态分布。使用Teros 12土壤水分传感器对同一样本进行了总孔隙率的测定。进行了四次测量。平均总孔隙率介于0.31至0.53之间，平均值为0.45，方差为0.0016。总孔隙率通常随深度（0.1-1米）降低，随后增加并随着深度加深趋于相对恒定的值。与水力传导率相比，总孔隙率的区域异质性在1-2米的深度最为显著，而水力传导率的区域异质性在较深的岩心样本中表现更为明显。