Hillslope groundwater discharges provide localized stream ecosystem buffers from regional per‐ and polyfluoroalkyl substances contamination

Abstract

Emerging groundwater contaminants such as per-and polyfluoroalkyl substances (PFAS) may impact surface-water quality and groundwater-dependent ecosystems of gaining streams. Although complex near-surface hydrogeology of stream corridors challenges sampling efforts, recent advances in heat tracing of discharge zones enable efficient and informed data collection. For this study, we used a combination of streambed temperature push-probe and thermal infrared methods to guide a discharge-zone-oriented sample collection along approximately 6 km of a coastal trout stream on Cape Cod, MA. Eight surface-water locations and discharging groundwater from 24 streambed and bank seepages were analysed for dissolved oxygen (DO), specific conductance, stable water isotopes, and a range of PFAS compounds, which are contaminants of emerging concern in aquatic environments. The results indicate a complex system of groundwater discharge source flowpaths, where the sum of concentrations of six PFAS compounds (corresponding to the U.S. Environmental Protection Agency third Unregulated Contaminant Monitoring Rule "UCMR 3") showed a median concentration of 52 ± 331 (SD) ng/L with two higher outliers and three discharges with PFAS concentrations below the quantification limit. Higher PFAS concentration was related (−0.66 Spearman rank, p < .001) to discharging groundwater that showed an evaporative signature (deuterium excess), indicating flow through at least one upgradient kettle lake. Therefore, more regional groundwater flowpaths originating from outside the local river corridor tended to show higher PFAS concentrations as evaluated at their respective discharge zones. Conversely, PFAS concentrations were typically low at discharges that did not indicate evaporation and were adjacent to steep hillslopes and, therefore, were classified as locally recharged groundwater. Previous research at this stream found that the native brook trout spawn at discharge points of groundwater recharged on local hillslopes, likely in response to generally higher levels of DO. Our study shows that by targeting high oxygen discharges the trout may thereby be avoiding emerging contaminants such as PFAS in groundwater recharged farther from the stream.

Continental-scale analysis of shallow and deep groundwater contributions to streams

“…Thus, streams with substantial shallow groundwater contribution are more vulnerable to extreme low flows or drying from climate change-related increases in drought or evapotranspiration, or from increased groundwater extraction. The high responsiveness of shallow groundwater to land surface disturbances also suggests streams with substantial shallow stream water contributions are likely more susceptible to diffuse nutrient and other pollution additions, while deeper groundwater can perpetuate legacy watershed land uses 3 and emerging contaminants such as per- and polyfluoroalkyl substances from outside the river corridor 4 . Still, shallow groundwater dominated streams may be more responsive to short-term management actions that reduce groundwater extraction and limit land application of fertilizers and other chemicals.…”

“…As a critical contributor to streamflow generation, groundwater discharge influences water quantity and quality throughout stream networks, especially during seasonal low flows and dry conditions 1 . Many streams host ecologically important ‘groundwater-dependent ecosystems’ 2 , yet these habitats face growing threats from climate change and groundwater contamination 1 , 3 , 4 . Aquatic organisms are particularly susceptible to shifts in thermal regimes because they have life cycles that rely on annual thermal cues 5 and metabolic rates influenced by stream temperature 6 .…”

Salmonid thermal habitat contraction in a hydrogeologically complex setting

“…6b and 7b). Similarly, it is difficult to discern what may be driving temporal variability in semivariance for flow-unconnected reaches; however, it certainly is driven by fluxes in surface water-groundwater interactions, which are underpinned by climatic fluxes and landscape controls (Dugdale et al 2013, Briggs et al 2020).…”

“…River temperature is a function of exogenic (surface) and endogenic (subsurface) thermodynamic processes, and the mixing of each, that is, surface water-groundwater interactions (Brodeur et al 2015, Briggs et al 2020. Exogenic processes are represented by climate and solar radiation fluxes (Caissie 2006, Dugdale et al 2017.…”

Evaluation of riverbed magnetic susceptibility for mapping biogeochemical hot spots in groundwater‐impacted rivers

“…These methods are typically time consuming, generating low spatial resolution data and cannot provide in situ information on the sediments. Various temperature methods, for example, fibre‐optic distributed temperature sensing (Briggs et al, 2012; Hare et al, 2015; Rosenberry et al, 2016), streambed temperature mapping (Briggs et al, 2020), and thermal infrared imaging (Hare et al, 2015) have been widely applied in rivers to help geolocate groundwater discharge zones that may be associated with Fe oxide precipitation. However, these techniques are generally based on the natural temperature contrast between groundwater and surface water and do not directly indicate biogeochemical hot spots.…”