Sustainable YIELD and why it matters

As the late Tom Weisner, former Mayor of Aurora and co-founder of the NWPA, explained in 2017, “[p]reserving the sandstone aquifers is critical because they're our insurance policy against a future water disaster. These aquifers provide emergency drinking water supply when a drought happens, or an algae bloom occurs.... Unless we act today to sustain our deep aquifers, we put our children—and their children—at great risk.”

All of the NWPA’s drinking water supplies provide an essential service to our communities, and the ability to continue to rely on each source is threatened by many challenges. Over a century of water withdrawals from northern Illinois’ deep sandstone aquifers has led to declining water levels, as more water has been withdrawn through pumping  than naturally replaced. Decades of activities on the land surface, such as the use of road salt and PFAs, has led to contamination of shallow groundwater and river water sources. Against this backdrop, the NWPA formed to collaboratively plan for and steward the shared river and groundwater resources, to ensure a sustainable water supply for the people, economy, environment, and future generations.  

From day one, the NWPA recognized the power of water conservation,  efficiency, and source water protection practices to sustain long term water supplies. However, one central question has evaded the NWPA until now -- just how much less water does the region need to use in order to maintain a water supply for generations to come? In order to answer this question, we first need to understand how much water we are currently using in relation to the sustainable yield.  

ISWS advances sustainable yield values and provides guidepost 
The Illinois State Water Survey (ISWS), with support from the Illinois Department of Natural Resources, will begin answering this question by releasing sustainable yield values for each water source by county later this year. Representing a significant advancement in groundwater knowledge, sustainable yield values will provide the NWPA with critical information needed to move the region to a sustainable path forward.  

What is sustainable yield? 
There is no one generally agreed upon definition or measure of sustainable yield. However, the concept of sustainable yield broadly represents how much water can be withdrawn, where and for how long, with acceptable physical, economic, environmental, social, cultural, institutional, and legal consequences. In this preliminary step, ISWS is first calculating sustainable yield for each water source based on available information and will layer on additional concepts in coming years to provide a more comprehensive 
assessment for communities.  

For the shallow aquifers, the physical sustainable yield is the volume of water entering the Sand and Gravel and Silurian Dolomite aquifers annually from natural recharge. The ISWS also plans to include an ecological component to recognize the importance of groundwater discharge to the streamflow of surface waterways and aquatic life. In the coming years, we can anticipate the sustainable yield calculations to be updated to refine this ecological component as we learn more about local conditions and include drought, climate change, and water quality considerations.  

The sustainable yield value for the sandstone aquifers will be calculated as the volume of water entering the aquifer basin annually minus the water leaving the basin, not including pumpage. The water entering and leaving the basin comes from a variety of sources: natural and anthropogenic recharge, and horizontal flow. Natural recharge is precipitation and streamflow that soaks into the ground and eventually travels down to the sandstone aquifers. Unintended anthropogenic recharge is water moving into and out of the sandstone aquifers via multi-aquifer wells. This type of recharge is included, because multi-aquifer wells are likely to persist given the expense and lack of a process to update them. Horizontal inflow and outflow includes water moving across the basin, generally from west to east. This is an important concept to include, as actions in a neighboring county can have an impact on water availability in another.  

Unlike the above water sources, Lake Michigan already has a legally-defined “water budget.” Set by the U.S. Supreme Court, Illinois has an official amount of water that it is allowed to withdraw from the lake.  Each community relying on this source is given an allocation of water as a condition of their permit. This sustainable yield estimate will account for the institutional aspects of sustainable yield, not the physical or ecological components.  

In a similar fashion, the Fox River sustainable yield value will not be defined by physical or ecological aspects, at least initially. In this first version, it will reflect the infrastructure capacity of existing users. An analysis done for Water 2050 recognized the potential of the Fox River to become a water supply source for additional communities. Therefore, we know that this initial sustainable yield calculation is a conservative estimate. Future updates will seek to include hydrological and ecological considerations, drought and climate change impacts, and water quality.  

Given that our water system is as dynamic as our communities, we anticipate that the sustainable yield values will be updated in the future as we learn more about our water resources, as our climate changes, and as the pattern of water demand shifts over time. In the coming years, ISWS will be adding other considerations of sustainable yield; refining these calculations will be an ongoing exercise. 

Using sustainable yield to inspire action 
Once derived, the sustainable yield value can be compared to current and projected water withdrawals to better understand the extent to which water use exceeds this sustainability metric. The region is anticipated to be withdrawing significantly more water from the sandstone and shallow aquifers than the sustainable yield value.  

The NWPA intends to explore the creation of a voluntary, regional sustainability goal to help move the area towards a sustainable path. The regional sustainability goal will be expressed as a percentage reduction from current and projected water use levels by water source needed to align with sustainable yield values.   

If annual water withdrawals from communities and industries equaled the sustainable yield value, then groundwater mining – or the depletion of stored water -- would be minimized in the sandstone aquifers, and groundwater discharge from the shallow aquifers to nearby streams would also be minimized. By reducing withdrawals, the current areas at risk of desaturation could recover, but by how much and how quickly will depend on local conditions. Additional considerations to sustainability could be added in the future to reflect community goals. 
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This broad goal will not be focused on any one community but will provide the NWPA region with an overall guidepost and a road map for water conservation and efficiency. To achieve regional sustainability, each water supplier/community will need to evaluate their own unique water supply assets and set their own goals for sustainability.  

Stay Tuned 
​In the coming months, the NWPA will be working with the ISWS to better understand the sustainable yield values and identify a voluntary regional sustainability goal. A sustainable water supply is critical for the future of our region; join the NWPA Technical Advisory Committee meetings to participate in the conversation.  ​