High Resolution Synoptic Salinity Mapping To Identify Groundwater–Surface Water Discharges in Lowland Rivers

Abstract

Quantifying distributed lateral groundwater contributions to surface water (GW-SW discharges) is a key aspect of tracking nonpoint-source pollution (NPSP) within a watershed. In this study, we characterized distributed GW-SW discharges and associated salt loading using elevated GW specific conductance (SC) as a tracer along a 38 km reach of the Lower Merced River in Central California. High resolution longitudinal surveys for multiple flows (1.3-150 m(3) s(-1)) revealed river SC gradients that mainly decreased with increasing flow, suggesting a dilution effect and/or reduced GW-SW discharges due to hydraulic gradient reductions. However, exceptions occurred (gradients increasing with increasing flow), pointing to complex spatiotemporal influences on GW-SW dynamics. The surveys revealed detailed variability in salinity gradients, from which we estimated distributed GW-SW discharge and salt loading using a simple mixing model. Modeled cumulative GW discharges for two surveys unaffected by ungauged SW discharges were comparable in magnitude to differential gauging-based discharge estimates and prior GW-SW studies along the same river reach. Ungauged lateral inlets and sparse GW data limited the study, and argue for enhancing monitoring efforts. Our approach provides a rapid and economical method for characterizing NPSP for gaining rivers in the context of integrated watershed modeling and management.

Synoptic Sampling to Determine Distributed Groundwater‐Surface Water Nitrate Loading and Removal Potential Along a Lowland River

“…We conducted the synoptic river surveys 17 times from 2010 to 2012 to examine longitudinal gradients using mobile in situ sensors. Pai et al () identified clear SC gradients in 13 of the 17 surveys, the surveys conducted at flows less than 32 m 3 s −1 . Greater flows resulted in overly diluted conditions for our analysis.…”

“…Due to the sparse (relative to our synoptic SW observations) and uneven distribution of GW SC data (Figure a), identifying an objective approach for assigning the groundwater concentration distribution along the river (used to estimate fluxes) was challenging. While our previous work involved an interpolated contour map based on inverse distanced weighting of the closest 12 wells (Pai et al, ), the well data screening discussed above resulted in a sparser spatial coverage (see supporting information Figure S4). After testing several averaging approaches, we assigned the GW concentrations in 1 km river segments, excluding wells located more than 5 km from the river, and averaging the concentration of the 3 wells closest to each segment.…”

“…We incorporated available GW well SC and nitrate data and our corresponding synoptic surveys into steady state mixing models to first estimate distributed GW‐SW discharge using SC (assumed to be conservative) and then examine for potential nitrate removal and estimate the average removal. Pai et al () covered the GW‐SW discharge model with SC at a 1 km resolution by applying mass balances for water flow and the specific conductance (SC). The flow mass balance assuming constant density and neglecting losses (i.e., evaporation) is expressed as where Q d , Q u , and Q g are downstream, upstream, and groundwater discharges (m 3 s −1 ) for a river segment.…”

Formation waters discharge to rivers near oil sands projects

“…Synoptic river chemistry surveys (e.g., Cook, ; Harrington et al, ; Pai, Villamizar, & Harmon, ; Smerdon et al, ) were completed along three rivers in the AOSR: the Christina River, the Clearwater River, and the Athabasca River.…”

“…Synoptic river chemistry surveys (e.g., Cook, 2013;Harrington et al, 2014;Pai, Villamizar, & Harmon, 2015;Smerdon et al, 2012) were completed along three rivers in the AOSR: the Christina River, the Clearwater River, and the Athabasca River. River water samples were collected from the surface, by hand, at n = 166 sites (n = 80 on the Clearwater and n = 86 on the Athabasca) for chemical analysis.…”