Comparison of Darcian flux calculations and seepage meter measurements in a sandy streambed in North Carolina, United States

Abstract

We compared two methods for estimating groundwater flux into a stream reach: seepage meters and Darcian flux calculations. Both methods were applied at the same 53 points on the streambed of a 62.5‐m‐long reach of West Bear Creek in the coastal plain of North Carolina. At each point, a seepage meter was used to measure groundwater flux, vmeter, and several minutes later the streambed hydraulic conductivity (K) and hydraulic head gradient (J) were measured. We compared the 53 paired point values of groundwater flux from each method (vmeter and vdarcy = KJ), the integrated volumetric groundwater flux to the stream reach from each method (Qdarcy and Qmeter), and the spatial distributions of vdarcy and vmeter on the streambed. Values of Qmeter (268 m3/d) and mean vmeter (0.66 m/d) were each about 0.70 × the corresponding Darcian flux quantities (Qdarcy = 375 m3/d and mean vdarcy = 0.92 m/d). Despite their differences, the two methods gave the same direction of groundwater flow (into the stream) at all 53 points, a result not found in all previous comparison studies and thus not to be taken for granted. Also, vdarcy and vmeter had similar spatial distributions on the streambed. The ratio vmeter/vdarcy = 0.70 is within the range of 0.3–7 from previous studies and is closer to 1 than in previous studies. The differences between the two methods are probably in part due to random measurement error and to the spatial scales for the Darcy and seepage meter measurements being of a slightly different size and offset several centimeters from each other on the heterogeneous streambed, but these effects are unlikely to produce the observed consistent bias. The mean bias between the methods (ratio of 0.70) may be related to gas bubbles in the streambed pore spaces (e.g., collection of gas bubbles in the seepage meters lowering vmeter and loss of gas from the streambed upon seepage meter removal or permeameter insertion increasing the subsequent vdarcy measurement).