Present and future thermal regimes of intertidal groundwater springs in a threatened coastal ecosystem

Abstract

Abstract. In inland settings, groundwater discharge thermally modulates receiving surface water bodies and provides localized thermal refuges; however, the thermal influence of intertidal springs on coastal waters and their thermal sensitivity to climate change are not well studied. We addressed this knowledge gap with a field- and model-based study of a threatened coastal lagoon ecosystem in southeastern Canada. We paired analyses of drone-based thermal imagery with in situ thermal and hydrologic monitoring to estimate discharge to the lagoon from intertidal springs and groundwater-dominated streams in summer 2020. Results, which were generally supported by independent radon-based groundwater discharge estimates, revealed that combined summertime spring inflows (0.047 m3 s−1) were comparable to combined stream inflows (0.050 m3 s−1). Net advection values for the streams and springs were also comparable to each other but were 2 orders of magnitude less than the downwelling shortwave radiation across the lagoon. Although lagoon-scale thermal effects of groundwater inflows were small compared to atmospheric forcing, spring discharge dominated heat transfer at a local scale, creating pronounced cold-water plumes along the shoreline. A numerical model was used to interpret measured groundwater temperature data and investigate seasonal and multi-decadal groundwater temperature patterns. Modelled seasonal temperatures were used to relate measured spring temperatures to their respective aquifer source depths, while multi-decadal simulations forced by historic and projected climate data were used to assess long-term groundwater warming. Based on the 2020–2100 climate scenarios (for which 5-year-averaged air temperature increased up to 4.32∘), modelled 5-year-averaged subsurface temperatures increased 0.08–2.23∘ in shallow groundwater (4.2 m depth) and 0.32–1.42∘ in the deeper portion of the aquifer (13.9 m), indicating the depth dependency of warming. This study presents the first analysis of the thermal sensitivity of groundwater-dependent coastal ecosystems to climate change and indicates that coastal ecosystem management should consider potential impacts of groundwater warming.

Closing the gap between science and management of cold‐water refuges in rivers and streams

“…Lateral seeps and springbrooks sourced by shallow groundwater may exhibit more variable and higher temperatures over time due to climate change (Hare et al, 2021;KarisAllen et al, 2022). Perennial streams also can become intermittent during extended dry periods (Costigan et al, 2015;Gendaszek et al, 2020;Price et al, 2021), thereby disconnecting CWPs from warm stream reaches in headwater systems (Briggs et al, 2022).…”

“…Groundwater discharge generates streamflow and influences stream thermal regimes because groundwater is generally cooler than surface water during the warm season and warmer in the cooler season (Bierkens & Wada, 2019). Lateral seeps and springbrooks sourced by shallow groundwater may exhibit more variable and higher temperatures over time due to climate change (Hare et al, 2021; KarisAllen et al, 2022). Perennial streams also can become intermittent during extended dry periods (Costigan et al, 2015; Gendaszek et al, 2020; Price et al, 2021), thereby disconnecting CWPs from warm stream reaches in headwater systems (Briggs et al, 2022).…”

“…Process‐based or numerical models have been applied at reach and watershed scales to identify thermally anomalous areas or reaches (Dzara et al, 2019; Fuller et al, 2021; Lee et al, 2020). Also, subsurface heat transfer models have been used to investigate the impacts of climate change on groundwater flow rates and temperatures at groundwater‐sourced CWRs (KarisAllen et al, 2022; Kurylyk et al, 2014); but no in‐channel surface mixing dynamics have been considered in these studies. To our knowledge, few studies have explicitly modeled the occurrence and dynamics of CWRs (Saadi et al, 2022; Wang et al, 2020).…”

“…investigate the impacts of climate change on groundwater flow rates and temperatures at groundwater-sourced CWRs(KarisAllen et al, 2022;Kurylyk et al, 2014); but no in-channel surface mixing dynamics have been considered in these studies. To our knowledge, few studies have explicitly modeled the occurrence and dynamics of CWRs(Saadi et al, 2022;Wang et al, 2020) Monk et al (2013).…”

Paired Air and Stream Temperature Analysis (PASTA) to Evaluate Groundwater Influence on Streams

“…Groundwater inputs can either impart thermal stability (deeper groundwater) or unexpected variability often due to effects of seasonal shifts on shallower groundwater (e.g., evapotranspiration and dry periods) (Benz et al., 2017; Briggs et al., 2018; Condon, Atchly, et al., 2020; Condon, Markovich et al., 2020; Hare et al., 2021). Shallow groundwater is more directly susceptible to land‐use changes (Kurylyk et al., 2015; Taniguchi et al., 2005), climate change (KarisAllen et al., 2022), and surface contamination (Cozzarelli et al., 2020), while older and deeper groundwater may experience natural contamination or contain contaminants from legacy land uses within a watershed (Ransom et al., 2022; Tesoriero et al., 2013). Also, natural chemistry varies laterally and with depth (Condon, Atchly, et al., 2020; Condon, Markovich et al., 2020; Zhi & Li, 2020), which has important implications for surface water quality and stream biogeochemical transformation.…”