Connectivity Processes
Surface water features such as lakes, wetlands and streams can have multiple sources of water. These include direct rainfall, runoff and groundwater. The interaction between surface water features and underlying aquifers can take place in different ways and in different settings. Stream reaches can interact with groundwater systems by (1) gaining groundwater inflow (2) losing water to the underlying aquifer or (3) both by variably gaining and losing depending on the time of year. Lakes can also gain from or lose to the underlying aquifer. A throughflow situation can occur where parts of the lake receive groundwater and other parts lose water. Some wetlands (such as fens) can form where groundwater discharges to the land surface, and these tend to occur at breaks in slope or topographic depressions (Winter et al. 1998).However, other wetlands (such as bogs) are surface water dominated and arise where rapid drainage of water from the land surface is prevented. Submarine groundwater discharge (SGD) refers to groundwater that discharges offshore into the ocean and can be particularly important for coastal geochemical and nutrient budgets. Also, groundwater discharge can be important for the ecology of estuaries due to relatively low salinity levels of groundwater as well as transported nutrients.
In connected water resources, the flow of water between the surface water feature and the aquifer is called the seepage flux. The convention is that positive seepage flux indicates upwards groundwater flow to the stream. Seepage flux can be an important part of the water budget for all surface water features. For example, this groundwater discharge is part of the baseflow regime of streams. This is different to the quickflow component of streamflow which relates to flow over the land surface (runoff) and rapid lateral movement in the soil profile (interflow). Perennial streams that flow continuously throughout the year have a high baseflow component.
Seepage flux is largely controlled by two factors. The first factor is the hydraulic gradient which reflects the difference between the water level in the surface water feature and the groundwater level in the aquifer. If the water level in the stream is higher than the surrounding watertable, then there is potential for the stream to lose water to the aquifer. Hence, processes such as stream regulation, water extraction or land use change which changes these water levels, can alter the connectivity. The second factor is the hydraulic properties of the aquifer, as well as the geological material separating the aquifer from the surface water feature. If a river has a coarse gravel bed, this would allow a high degree of interaction between the river and the underlying aquifer, as gravels can transmit water efficiently. It is important to note that groundwater-surface water interactions can operate at different scales in time and space.
There are many different approaches to categorising the hydraulic connectivity of streams and aquifers. Generally classification can be in terms of whether there is direct hydraulic connection, the direction of the seepage, the hydraulic properties of the aquifer and the potential impact of the connectivity on water management.
Further Information
The hydrological cycle
Connectivity examples
Seepage flux
Baseflow
Factors controlling connectivity
Scale issues in connectivity
Defining connectivity categories
Relevant Links
Groundwater/Surface water Interface (GSI)
US Geological Survey Ground Water and Surface Water, A Single Resource
US Geological Survey Ground Water - Surface Water Interactions
US Geological Survey Sustainability of groundwater resources
Environment Canada The Nature of Water
The UK Groundwater Forum
References
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/