Using Boreholes as Windows into Groundwater Ecosystems

Abstract

Groundwater ecosystems remain poorly understood yet may provide ecosystem services, make a unique contribution to biodiversity and contain useful bio-indicators of water quality. Little is known about ecosystem variability, the distribution of invertebrates within aquifers, or how representative boreholes are of aquifers. We addressed these issues using borehole imaging and single borehole dilution tests to identify three potential aquifer habitats (fractures, fissures or conduits) intercepted by two Chalk boreholes at different depths beneath the surface (34 to 98 m). These habitats were characterised by sampling the invertebrates, microbiology and hydrochemistry using a packer system to isolate them. Samples were taken with progressively increasing pumped volume to assess differences between borehole and aquifer communities. The study provides a new conceptual framework to infer the origin of water, invertebrates and microbes sampled from boreholes. It demonstrates that pumping 5 m3 at 0.4–1.8 l/sec was sufficient to entrain invertebrates from five to tens of metres into the aquifer during these packer tests. Invertebrates and bacteria were more abundant in the boreholes than in the aquifer, with associated water chemistry variations indicating that boreholes act as sites of enhanced biogeochemical cycling. There was some variability in invertebrate abundance and bacterial community structure between habitats, indicating ecological heterogeneity within the aquifer. However, invertebrates were captured in all aquifer samples, and bacterial abundance, major ion chemistry and dissolved oxygen remained similar. Therefore the study demonstrates that in the Chalk, ecosystems comprising bacteria and invertebrates extend from around the water table to 70 m below it. Hydrogeological techniques provide excellent scope for tackling outstanding questions in groundwater ecology, provided an appropriate conceptual hydrogeological understanding is applied.

Identifying priorities for nutrient mitigation using river concentration–flow relationships: The Thames basin, UK

“…The constant phosphorus inputs will primarily equate to STW point source inputs to the River Thames, but will also include some groundwater inputs, although these will be relatively small. P concentrations in Thames basin groundwaters are low (<20 µg l -1 ) due to P precipitation within the Chalk geology (Neal et al, 2002;Sorensen et al, 2013). The patterns in the phosphorus concentration-flow relationship had a lot of scatter, especially at intermediate flows, indicating either the presence of intermittent phosphorus pollution events that were neither constant nor rain-related (Jordan et al, 2007), or there were large hysteresis patterns during some storm events.…”

Towards an integrated understanding of how micro scale processes shape groundwater ecosystem functions

“…Another issue is that the groundwater well environment provides associations that are different to those in the surrounding groundwater in terms of metazoa (Korbel et al, 2017;Matzke and Hahn, 2005;Sorensen et al, 2013;Steenken, 1998), protozoa (Korbel et al, 2017), and bacteria (Korbel et al, 2017;Roudnew et al, 2014;Sorensen et al, 2013). The best recommendation to date is to combine methods in the least disturbing way in order to complement results.…”

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…, either suggesting disequilibrium with calcite or perhaps additional P sources from mains water leakage (Gooddy et al, 2015). Sorensen et al (2013) compared bulk borehole and aquifer chemistry and ecology using packer sampling to show that the borehole environment was depleted in SRP and enriched in PP, DHP, (as well as NO3 and DOC) with respect to the aquifer water, implying that there was localised biological cycling of macronutrients within the borehole environment.…”