Now that all the snow is about to melt and eventually run down the Mississippi River to the Gulf of Mexico, it’s a good time to wonder whether there’s something else that should happen to that water and all the other precipitation that falls on Minnesota. Could spring floodwaters be captured and used later? Should stormwater be injected into underground aquifers, as is done elsewhere?
As usual when it comes to water, things can quickly get complicated. These four water experts sort it out in our fifth water Q and A.
FROM THE PUBLIC INSIGHT NETWORK: Is there a way to use stormwater to recharge groundwater aquifers?
Ali Elhassan, water supply planning manager, Metropolitan Council
Absoutely! Stormwater can recharge aquifers through infiltration basins, such as stormwater ponds. This is a common practice and is an option being evaluated as part of the Council’s water supply study. However, there are several considerations:
• There must be sufficient volume of stormwater to sufficiently contribute to groundwater recharge.
• Soils must be permeable enough to absorb the volume of stormwater.
• The water table must be far enough below grade to avoid flooding.
• Stormwater treatment may be needed to avoid contaminating the aquifer.
There clearly are ways to use storm water to recharge groundwater aquifers, such as properly designed rain gardens and other infiltration practices, but it is not as simple as one might think. Storm water is not currently managed to treat water quality impairments that happen when excess nutrients, chemicals, pesticides, heavy metals and petroleum products get added to the rainfall that runs off the landscape. Also, if we talk about using stormwater for other purposes, such as landscape irrigation, it may not be recharging groundwater, but it can be used to offset the need to pump groundwater.
The degree of treatment that would be necessary to introduce storm water into our aquifers, which we depend on for drinking water supplies, will present serious challenges in engineering and cost in most cases. There are stringent public health standards for drinking water and we are constantly concerned about the quality of water that might be introduced into our aquifers.
There might be even better, more cost-effective solutions for recharging our aquifers, than storm water, that also need to be explored. These solutions will come from a change in attitude about how we manage our land and water that will help prevent quantity and quality problems created by our current infrastructure designs and land use management practices.
Stormwater infiltration is taking place in Minnesota in various forms, primarily as a water-quality measure. Many watershed districts in the Metro area require that new developments have infiltration basins to infiltrate the first 1.5 inches of a storm so that this runoff (which typically contains land-based pollutants) does not flow directly into streams. Rain gardens are a smaller version of that same concept. While water quality has been the driver, it has the added benefit of increasing infiltration and thereby increasing groundwater recharge. Storm water infiltration basins increase recharge to the water table (compared to, say, an undeveloped land surface) because the runoff is concentrated into a single place with (a) a driving head downward and (b) increased soil moisture between the bottom of the pond and the water table, which substantially increases the rate of movement of water.
There are some water quality concerns about increasing small-point-source infiltration of storm water. To date, there is little evidence that storm water infiltration basins are adversely affecting groundwater quality, although there are some studies that indicate it could, over time, increase the temperature of groundwater (which has some implications for cold-water fisheries, such as trout streams).
Interestingly, there is growing evidence (including modeling we have done) that indicates that as agricultural and undeveloped land is urbanized (meaning removal of substantial vegetation, increased impervious area, and infiltration basins) groundwater recharge actually increases. This is due to (a) focused recharge (as described above) and (b) reduced evapotranspiration. During the months of June through August, there is hardly any recharge at all to the groundwater system because of evapotranspiration – nearly all of the precipitation that reaches the ground surface is used by plants.
The real game changer for water supply in Minnesota and the metro area in particular will be the capture and storage of flood waters during spring melt. This is the only reliable time of the year when there is a huge excess of available water. The problem is how to store this water and how to treat it. Treatment is straightforward – we know how to treat surface waters. Storage is more challenging. There are two basic storage options – ground reservoirs and underground geologic storage (i.e. in aquifers). Injecting storm water during floods (either treated or untreated) is currently not allowed in Minnesota (but infiltrating it is allowed, generally). But injection is a very common technology in the western states (ASR for Aquifer Storage and Recovery). The one ASR project we have in Minnesota only uses water pumped from the same aquifer, treated, and then reinjected (it balances out the water treatment plant but does nothing to affect the overall water balance or availability). If we, in Minnesota, were to employ ASR, we would basically solve all of our groundwater and surface water quantity problems, now and into the future. Unless there is a dramatic change in climate, we can rely on large surpluses of water during spring freshet.
One last thought – some are prescribing using more surface water in parts of the metro area to replace groundwater usage. That generally makes sense but it also means that larger portions of the metro area will become dependent on a single source of water (the Mississippi River). The Mississippi River is vulnerable to drought. During drought conditions, even more of the metro area than the current core cities would be at risk. A system that injects flood water for storage in ground systems for withdrawal during times of drought would greatly reduce that vulnerability.
Yes, recharging storm water or treated waste water is done in many areas of the country, particularly in the arid west. In some areas of Minnesota, treated waste water and storm water are allowed to percolate into the ground as a final method of treatment. Local-site conditions need to be appropriate for this process to work efficiently. It is likely that the wide-scale recharge of storm water, or waste water, will require a change in public perception in order to be widely accepted in the water-rich Midwest.