Connectivity examples
This website has a focus on stream-aquifer connectivity because of its importance in the management of water quantity and quality in Australian catchments. However, the general principles of interaction with groundwater systems equally hold for oceans, estuaries, wetlands and lakes. Here are some other examples of connectivity:
River baseflow systems
Gaining streams where the connected aquifer contributes to streamflow are also called river baseflow systems (Boulton and Hancock, 2006). The discharge of groundwater can have significant implications in terms of the flow regime, water quality and the aquatic ecosystems of the stream.
Lakes and Reservoirs
Lakes interact with underlying aquifers in a similar way to streams, however the time scales of interaction may differ. For example, the water level in natural lakes tends to be more stable, lakes are more subject to evaporative losses and the generally low permeability of lake sediments can affect the exchange of water and solutes (Winter et al, 1998).
Subterranean groundwater discharge (SGD)
SGD is groundwater that discharges offshore via either unconfined or confined aquifers. In areas where stream flow is low, this may dominate freshwater discharge and so control the distribution of freshes, nutrients and contaminants in the marine environment. This is particularly important for coastal geochemical and nutrient budgets. Geochemical studies reveal an array of chemical reactions in the mixing zone between fresh water from coastal aquifers and seawater. These reactions can be related to carbonate diagenesis and dolomite formation. Sites of submarine groundwater discharge can also form unique ecosystems on the sea floor.
Coastal groundwater discharge
Groundwater discharge to estuaries or the beach shoreline occurs where the aquifer saltwater-freshwater interface intersects the land surface. Such discharge may be important ecologically as a measure of the freshwater component itself as well as the nutrients transported and supplied to coastal surface waters. Such seepage can also be a pathway for contaminants to the coastal environment. For example, the release of shallow acid groundwater generated in areas of acid sulfate soils is a significant coastal management issue.
Salt-water intrusion
Salt-water intrusion occurs where excessive extraction of groundwater causes drawdowns that allow seawater to migrate landwards in the coastal aquifer. This results in the salinisation of the groundwater resource, rendering drinking water non-potable as well as promoting ion exchange and other chemical reactions. Channel dredging, by breaching confining layers, can increase the contact between seawater and groundwater and intensify the problem. Sea level rise, waves and tides may also exacerbate this effect.
Tidal fluctuations
Fluctuations in groundwater levels in the coastal zone can be caused as far as 100m inland by both waves and tides. Beach slope and the drainage characteristics of the sand also help determine the degree of interaction. The height of the watertable driven by these factors affects the stability of structures built on overlying surfaces, beach sediment transport, and salt-water intrusion.
Groundwater dependent wetlands
Wetlands can rely on permanent or seasonal groundwater discharge to maintain their waterlogged or wet status. They may be wholly reliant on groundwater flow for maintenance (eg. mound springs in the Great Artesian Basin) or partially reliant. Systems of the former type can show rapid responses to reductions in groundwater flow (such as loss of species diversity and spatial area). Also due to the often unique chemical composition of groundwater, in highly dependent wetland systems unique faunal assemblages that have adapted to these conditions can be threatened not only by changes to volume, but by any significant change in quality.
Figure 1: Idealised freshwater/saltwater interface influenced by submarine groundwater discharge (Swarzenski et al, 2004)
Case Studies
Subterranean groundwater discharge (SGD) - South Atlantic Bight, USA
Coastal groundwater discharge - Leschenault Inlet
Salt-water intrusion - Mackay coastal plains
Tidal fluctuations - Barrenjoey Beach
Relevant Links
US Geological Survey Role of lakes in the hydrologic system
IAHS-IAPSO Joint Commission on Groundwater-Seawater Interactions
Land-Ocean Interactions in the Coastal Zone (LOICZ)
Seawater Intrusion FAQ
SaltNet - Saltwater intrusion and coastal aquifers
References
Boulton AJ, Hancock PJ, 2006. Rivers as groundwater-dependent ecosystems: a review of degrees of dependency, riverine processes and management implications. Australian Journal of Botany 54, 133-144.
Church TM. 1996. An underground route for the water cycle. Nature 380: 579-580.
Moore WS. 1999. The subterranean estuary: a reaction zone of ground water and sea water. Marine Chemistry 65: 111-125.
Winter, TC, Judson, WH, Franke, OL, Alley WM. 1998. Groundwater and surface water a single resource. Circular 1139, U.S. Geological Survey, Denver. http://pubs.usgs.gov/circ/circ1139/
Swarzenksi PW, Charette M, Langevin C, 2004. An autonomous electromagnetic seepage meter to study coastal groundwater/surface water exchange