The Importance of Groundwater in Critical Zone Science

Abstract

The critical zone (CZ)-from treetops to groundwater-is an increasingly studied part of the earth system, where scientists study interactions between water, air, rock, soil, and life. Groundwater is both a boundary and an essential store in this integrated system, but is often not well considered in part because of the difficulty in accessing it and its slow movement relative to other parts of the system. Here, we describe some fundamental areas where groundwater hydrology is of fundamental importance to CZ science, including sustaining streamflow and vegetation, reacting with minerals to produce dissolved solutes and regolith, and influencing energy fluxes across the land-atmosphere interface. As the timing and type of precipitation change with climate, groundwater may play an even more important role in CZ processes as a sustainable water source for plants and streamflow. Many open questions also exist about the role of CZ processes on groundwater. Many data streams are needed and important to quantifying the integrated response of the CZ to groundwater and vice versa, but long-term data records are often incomplete or discontinued due to limited funding. We argue that the long timescales of processes that involve groundwater necessitate data collection efforts beyond typical federal funding timespans. Sustaining monitoring networks and developing new ones aimed at testing hypotheses related to slow-moving, groundwater-controlled CZ processes should be a scientific priority, and here we outline some open questions that we hope will motivate groundwater scientists to get involved in CZ science.

Groundwater deeper than 500 m contributes less than 0.1% of global river discharge

“…continental-scale glaciation, downcutting by a large river) connects these deeper fluids with the surface. Understanding the links between deep groundwater and streamflow is fundamental to surface water hydrology and the field of critical zone science, which uses the bottom of groundwater as its lower limit 19,20 without a clear definition of how this depth relates to hydrological and biogeochemical cycles.…”

Bedrock depth influences spatial patterns of summer baseflow, temperature and flow disconnection for mountainous headwater streams

“…Mountain headwater stream habitat is affected by hydrologic connectivity along the surface channel, and connectivity between the channel and multiscale groundwater flowpaths (Wohl, 2017;Covino, 2017;Fausch et al, 2002). Discharge from shallow groundwater within the critical zone is a primary component of stream baseflow, attenuating maximum summer temperatures and creating cold water patches (Singha and Navarre-Sitchler, 2021;Sullivan et al, 2021), and shaping catchment topography (Litwin et al, 2022). In headwater stream valleys characterized by irregular bedrock topography and thin, permeable sediments, nested physical processes interact to control the connectivity of groundwater/surface water exchange (Tonina and Buffington, 2009).…”

Elemental Stoichiometry (C, N, P) of Soil in the Wetland Critical Zone of Dongting Lake, China: Understanding Soil C, N and P Status at Greater Depth

“…Liu et al [24] integrated the mechanisms of various driving forces to give a conceptual model of wetland critical zones and an overall framework for the coupling of water, soil, rock and biology in wetland ecosystems, with spatial boundaries ranging from the vertical area of the wetland vegetation canopy (above) and the base of the aquifer (below). Such a framework suggests that the wetland critical zone covers a wider area and is vertically thicker than the surface wetland zone, the surface water-groundwater interaction zone, and the vertical infiltration zone, with more complex nutrient biogeochemical cycles, patterns and multiple influences, and where elemental transport, exchange, cycling and interaction responses are more active [25,26]. Therefore, by studying the changes in C, N and P contents in wetland critical zone soils and their ecological chemometric characteristics, we can more comprehensively reveal the drivers of nutrient transport and transformation in wetland ecosystems, and thus explore the biogeochemical cycling of nutrients and their coupling mechanisms.…”