Connectivity Processes

Factors controlling connectivity

The flow of water between a surface water feature and the underlying aquifer is largely controlled by:

1. The difference between the surface water level and the groundwater level. Water moves down gradient from high potential to low. If the stream level is higher than the groundwater level measured within the aquifer, then the stream has the potential to lose water to the aquifer. This is indicated in Figure 1 where negative seepage flux occurs because the shallow watertable is lower than the stream stage. Conversely, for groundwater to discharge into a stream channel, the elevation of the groundwater surface near the stream must be higher than the elevation of the river stage, such as in Figure 2.
2. The hydraulic properties and features 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. If the base of a lake consists of a thick sequence of clay, this is likely to restrict movement of water.

Figure 1: A losing stream where stream levels are higher than the surrounding watertable (Winter et al, 1998)

Figure 2: A gaining stream where stream levels are lower than the surrounding watertable (Winter et al, 1998)

These are the controlling factors because the direction and magnitude of seepage flux is based on Darcy's Law:

Equation 1 Q/A=(dh/dl)*K

where Q is the flow of water (volume per unit time), A is the cross-sectional area of the porous medium through which flow occurs (eg the stream bed), dh/dl is the hydraulic gradient where dh is the change in hydraulic head (or measured water level) along the distance dl of the groundwater flow line, and K is the hydraulic conductivity of the material. Groundwater can move towards the stream if the hydraulic gradient is positive in the sense that the groundwater potential further from the stream is higher than that closer to the stream. The parameter K describes the rate at which water can move through the geological material, and this is largely defined by texture. Coarse grained sediments such as gravel or sand have a high hydraulic conductivity so can readily transmit water, in contrast to much lower flow rates in finer grained material such as silt or clay. Also hydraulic conductivity can vary significantly with the direction of water flow. The hydraulic conductivity in the vertical direction (Kv) in aquifers can be several orders of magnitude lower than that in the horizontal direction (Kh). This is because the interlayering of finer-grained clays and silts impedes vertical water movement, while laterally extensive sand and gravel deposits enable high rates of horizontal flow.

Hence, the rate of groundwater movement near the stream is dictated by variations in hydraulic conductivity of the surrounding geological material. This can lead to preferential groundwater flowpaths within more transmissive parts of a sedimentary sequence, as indicated in Figure 3. In alluvial sequences, paleochannels infilled with coarse sediments can provide opportunities for localised enhanced connectivity. In contrast, sequences of low conductivity material such as clays and mudstones can act as barriers for groundwater flow, becoming aquitards or confining beds. These can impede the flow of groundwater from (or to) any deeper confined aquifers. Although, streams are commonly associated with a floodplain of alluvial sediments, in upland areas there may be direct contact with groundwater systems developed in fractured rock systems. In this setting, preferential flowpaths can be developed in zones of intense fracturing and/or weathering. In karstic terrains, carbonate dissolution can result in extreme interactions of conduit flow where the stream disappears into the aquifer to re-emerge further downstream. Seepage flux can also be controlled by the hydraulic properties of the stream bed as well as the properties of the surrounding aquifer. Deposits on the stream bed such as mud veneers or algal mats can significantly impede the flow of groundwater.

Figure 3: Development of springs due to preferential groundwater flow in sand horizons (Winter et al, 1998)

As seepage flux depends on the relativity between stream and groundwater levels, the factors that affect these water levels will also affect connectivity. Groundwater-surface water interactions can change in time and space in response to natural factors such as climate. However, catchment development and management can also significantly change stream-aquifer connectivity over time. Specific activities that can influence connectivity include:

1. Stream regulation where flow is controlled by infrastructure such as dams, locks or weirs. Releases from surface water storages for downstream users can make up the bulk of streamflow and artificially maintain a high stream stage during dry periods. This can have the effect of reducing inflows in gaining reaches or increasing stream losses in losing reaches.
2. Return Flows can artificially maintain stream stage during dry periods. These include direct discharges, such as from sewage treatment plants, industrial outfalls, power generating facilities, mine dewatering activities or seasonal return flows from drainage of irrigation areas.
3. Surface water extraction from the stream for consumptive uses such as irrigation, urban supply or industry can lead to lowering of the stream stage relative to surrounding groundwater levels. This includes artificial diversion of water out of the stream as part of inter-basin transfer schemes.
4. Groundwater extraction can be sufficient to lower the watertable and decrease or reverse the hydraulic gradient towards the stream. Interactions can be altered due to pumping where a gaining stream receives reduced discharge due to interception of groundwater that would have naturally entered the stream (Figure 4b). In some cases, groundwater pumping can reverse flowpaths near the stream (Figure 4c). This latter scenario is referred to as induced recharge as the stream becomes losing.
5. Land development, such as urbanisation or irrigation development can significantly alter the water budget in the floodplain. Artificial drainage can induce rapid runoff and reduce aquifer recharge. This can lower the watertable and alter the hydraulic gradient within the stream. Irrigation accessions can do the opposite and cause the watertable to rise.
6. Land cover change, such as clearing, reforestation or replacement of crop type, can significantly alter evapotranspiration rates. For example, broadscale clearing of native vegetation types that are efficient interceptors of soil water has resulted in greatly enhanced recharge in parts of Australia. This is reflected in higher water tables and increased influx of groundwater to streams. In areas of shallow saline groundwater this has increased stream salt loads. Conversely, plantation forestry can increase plant water use and reduce the baseflow component of streamflow.
7. Engineering or mining activities such as straightening and lining of the stream can drastically alter connectivity. Such works are commonly undertaken in urban areas and can isolate the stream from the aquifer. Levees for flood control can change how and where seepage to the aquifer occurs during high flow events. Also, alluvial mining and dredging activities can alter stream geomorphology by deepening channel incision or removing or mixing bed deposits.
8. Global warming due to increased levels of gases such as carbon dioxide and methane in the atmosphere is projected to change the magnitude and variability of rainfall across Australia. This will have impacts across the water budget for catchments, including surface runoff, groundwater recharge and seepage flux.

Figure 4: Effects of groundwater pumping on river-groundwater interaction showing (a) natural groundwater discharge (b), reduced discharge (b) and (c) induced recharge conditions, after Winter et al, (1998)

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/