Ecological dynamics in the riverine aquifers of a gaining and losing river

Abstract

The effects of river-flow fluctuations on groundwater ecosystems and differences in ecological conditions in aquifers connected to losing, gaining, and alternating river reaches are poorly known. We used monitoring wells at gaining and losing reaches along an alluvial river to identify associations between river and groundwater fluctuations and ecological conditions in riverine aquifers. Groundwater and surface-water pH, temperature, and dissolved O 2 (DO) were positively correlated at all sites, and groundwater and surface-water dissolved organic C (DOC) concentrations were positively correlated at 3 sites. Groundwater and surface water in the gaining section were more NO 3 --enriched and highly mineralized than in the losing section. These differences reflect the primary water source in each section. The gaining section is dominated by regional groundwater and the losing section by hillslope runoff. The groundwater-surface-water correlations and comparisons between sections indicate that physicochemical conditions in losing-reach aquifers are controlled by surface-water infiltration, and conditions in gaining-reach aquifers are controlled by groundwater inflow from up-gradient aquifers. At some sites, groundwater DOC concentrations were positively correlated with DO concentrations and heterotrophic bacterial densities. These correlations suggest that DOC and DO are depleted simultaneously by aerobic microbial processes and that bacterial populations are DOC-limited, which may in turn limit invertebrate abundance. DOC concentrations, bacterial densities, and invertebrate density and taxon richness varied in response to river flow, groundwater levels, and time since floods at some sites. Patterns of river-aquifer exchange and hydrology-groundwater-ecology relationships are prerequisites for identifying the river-flow requirements for healthy groundwater ecosystems.

Groundwater–surface-water interactions: current research directions

“…The 1 st issue has been studied for decades in groundwater-dominated rivers, but a persistent shortage of information exists about groundwater ecosystems. Larned et al (2015) addressed that shortage with a study of groundwater-ecosystem responses to riverflow variation along an alluvial river. The authors compiled groundwater and surface-water hydrological, chemical, and biological data for 5 y from losing, variable, and gaining river reaches.…”

Hyporheic exchange controlled by dynamic hydrologic boundary conditions

“…As a first step toward representing dynamic hydrologic forcing at the seasonal scale, several studies have explored exchange alterations at different base flow conditions [Payn et al, 2009;Voltz et al, 2013;Ward et al, 2012]. Interactions between seasonal hydrologic changes and catchmentscale geomorphic setting (e.g., topography and topology) control the spatial distribution and timing of stream-hillslope connectivity across a watershed [Godsey and Kirchner, 2014;Jencso et al, 2009Jencso et al, , 2010Larned et al, 2015;McGuire and McDonnell, 2010;Wroblicky et al, 1998]. Shifts in timing and magnitudes of exchange are also controlled by interactions at smaller temporal and spatial scales.…”

Groundwater–surface-water interactions: perspectives on the development of the science over the last 20 years

“…Groundwaterrelated studies in the 2015 special series range over spatial scales from local streamside aquifers to regional aquifers (Table 1). For example, seepage through the beds of rivers can recharge large aquifers so that distance from the river may be a primary influence on groundwater chemistry and the distribution of organisms found in these groundwater systems (e.g., Larned et al 2015). Conversely, aquifer recharge may be dominated by precipitation so that spatial patterns in the overlying soils and regional geology may determine the distribution of organisms found in groundwater systems (Johns et al 2015, Korbel andHose 2015).…”

“…Studies at the scales of large floodplain aquifers to regional aquifers are difficult because access to these systems requires deep wells. As a consequence, such studies are relatively rare and those published here are mostly descriptive (Johns et al 2015, Korbel and Hose 2015, Larned et al 2015. Several authors in the 2015 special series explored methodological approaches to studying GW-SW interactions (study topic = M or S/M, Table 1).…”