Pop Quiz: What is a good depth to groundwater for an alluvial recharge site?
A) Less than 8 feet B) Between 8-10 feet C) Greater than 10 feet
Answer: C. Greater than 10 feet
Typically, alluvial recharge sites are used by larger entities with multiple water rights and multiple return flow obligations, although smaller entities also use alluvial recharge sites. The reasons for utilizing alluvial recharge can be numerous, but the science behind how to do it is established. Here some of the top characteristics we look for when assessing a good alluvial recharge site.
1. Depth to Groundwater at the Alluvial Recharge Site Is Greater Than 10 Feet
Why? Not a lot of plants have roots deeper than 10 feet. Even fewer have roots deeper than 15 feet. When the water is deeper than root depth, it prevents that water from being consumed by plants which would reduce the amount of water that will eventually return to the surface stream system. To preserve as much of your recharge credit as possible, sites with a depth to groundwater greater than 15 feet are best.
2. Depth to Water Near the River or Stream Is at Least 10 Feet
This is a very similar concept to the first point. Recharge accretions may be available to increasing numbers of plants along floodplains if the depth to water is less than 10 feet. This will result in a reduction of recharge credit. A steeper sided river valley will result in less area for the water to be available to the root zone.
3. There’s a Hydraulic Connection Between the Alluvial Recharge Site and River or Stream
There needs to be a continuous alluvial groundwater aquifer from the recharge site to the river or stream. Otherwise, recharge credit cannot be realized, as the water isn’t returning to the surface stream system.
4. The Area Under the Alluvial Recharge Site Is Composed of Highly Conductive Material
A material that water can quickly and easily travel through minimizes groundwater mounding around the recharge pond. This allows more of the recharge water to get to the aquifer and less to vegetation surrounding the pond. For example, sand is a fast material for water to move through (conductive) and clay is slow (not conductive). More highly conductive materials enable the site to recharge more water to the groundwater aquifer. Less conductive material causes recharge water to spread out laterally, resulting in more recharge water being consumed by plants and less water getting to the groundwater aquifer.
5. Distance From the Alluvial Recharge Site to the River or Stream
There are numerous factors that impact the timing of the return flows from the recharge site, and distance is a big one. Larger distances result in longer return flow timing.
6. Path Between the Alluvial Recharge Site and the River or Stream Is Free of Obstacles
Not only does a hydraulic connection need to be assured between the recharge site and the surface stream system, but that path also should be free of obstacles that could compromise the flow path or reduce the volume of recharge water that reaches the river or stream. For example, obstacles such as reservoirs, ditches, or areas that have shallow depth to water could cause the interception or consumption of some of the recharge water.
Recharging water to the alluvial aquifer is an important part of water rights management. To learn more about selecting a good alluvial recharge site or to discuss this further, contact us today.