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Surface Water-Groundwater interaction
One specific class of inflow or outflow from groundwater systems results from surface water–groundwater interaction, wherein water may flow from aquifers into surface water bodies at seeps or springs, or infiltrate from rivers or lakes to the water table (Figure 39; also note the dual-sided arrow between the aquifer and stream in Figure 37 indicating that the flux may be either to or from groundwater to surface water). If there is a net groundwater flux to surface water, the surface water body is said to be gaining (for example, a gaining stream is one that is fed by groundwater). As you may recall from Module 4, the component of streamflow derived from groundwater influx is termed baseflow. In this scenario, surface water-groundwater interaction leads to an outflux from the groundwater system. Alternatively, if the water table lies below the surface water body, the potential energy (hydraulic head) in the surface water body will be higher than in the groundwater system and water will percolate downward and/or through the stream banks, and flow to the aquifer. In this configuration, the surface water body is said to be losing (i.e. a losing stream), because the stream or river discharge decreases downstream. While the land surface and stream channel generally remain at the same elevation, the water table commonly fluctuates over time (c.f. Figures 32-33). As a result, it is common for streams to switch from gaining to losing – or vice-versa – due to major recharge events, seasonality in precipitation and recharge, and variations in pumping rates.
Although water rights and policies are in some cases constructed with the implicit assumption that surface water and groundwater systems act independently, this is clearly not the case. A number of interesting situations arise from their interaction. As noted above in the Effects of Pumping Wells section, pumping at wells can reverse groundwater flow, and change a gaining stream to a losing one. In such a scenario, it isn't always clear whether surface water rights are violated by groundwater pumping – even though groundwater extraction directly causes reduction in surface water discharge, the water is withdrawn from the groundwater system, not the river. In large aquifer systems, the intercepted baseflow may impact users far downstream, across county and state borders. In other cases, also as noted earlier in this module, substantial or rapid influxes of surface water to groundwater systems, for example through fractures or sinkholes, raise the specter of groundwater contamination. If direct connection between surface water and groundwater is demonstrated by the presence of microorganisms or increased water turbidity (cloudiness indicating suspended particles) in well water, additional treatment of groundwater is required before it is considered suitable for domestic or municipal use.
Source: Colorado State Division of Water Resources
Source: USGS Circular 1186