Post by Eric Booth
As part of the Yahara WSC project, we are very interested in understanding the journey of water through the watershed from precipitation, across different land types, underground, through streams, etc. One component of this journey is the one that we interact with the most on a daily basis: the process of getting drinking water into our homes and workplaces.
Here in the Yahara watershed, we are very fortunate to have an abundant supply of fresh and clean water lying below our feet in an aquifer known as the Mount Simon sandstone. This 300-700 feet thick rock layer is very permeable and saturated with water. Thus it can yield an abundant supply of water to wells that drill down deep enough to reach it. If you get your water from a municipality in the Yahara watershed, it is very likely coming from this sandstone aquifer. (To further explore the Yahara’s geology, please see our previous posts)
However, the Mount Simon aquifer is not the only aquifer that we have in Madison. Groundwater can also be found in shallow glacial material and in the upper bedrock aquifer. This upper aquifer interacts directly with the land surface and the top of this aquifer, or the water table, intersects with the surface of streams and lakes throughout the watershed. This shallow groundwater in the upper aquifer can also interact with vegetation, and WSC scientists are studying the impacts of shallow groundwater on corn production in the Yahara.
Between this upper aquifer and the lower Mount Simon sandstone aquifer lies the Eau Claire shale, which is a relatively impermeable layer known as a confining unit because water moves very slowly through it. This is a fortunate arrangement for Yahara residents, because it prevents most of the water in the upper aquifer—which can be contaminated by road salt, gasoline leaks, industrial pollution, etc.—from moving into the lower aquifer and the public water supply.
Unfortunately, this confining unit is not present everywhere. Wells that are open both above and below it can act as a short circuit for water to mix between aquifers. Water from the upper aquifer can enter the well and then migrate down the pipe and discharge into the lower aquifer. This has led to recent concerns about low levels of contamination at some wells around the Madison metropolitan area.
The Madison area has been pumping the Mount Simon sandstone aquifer for its water supply since the late 19th century. In the early days of the city, shallow wells provided water for drinking and industrial purposes. Unfortunately, this was also a time before sanitation infrastructure was developed, and raw sewage was usually disposed in privies throughout the city that could leak into groundwater. In 1880, a UW-Madison geology student (Magnus Swenson) found that nearly all of the backyard wells that he tested were contaminated with sewage and were likely connected to cases of typhoid fever, scarlet fever, and diphtheria.
Facing a major public health problem, the city of Madison began construction of a public water supply system in May of 1882 and started service by December of that year. Demand grew very quickly in the first few years, and the new public water works was almost universally celebrated. Lake Mendota would periodically become a candidate to help satisfy increasing demand but the city and its residents were rightly skeptical of its quality for drinking purposes due to ongoing sewage issues (this topic will be covered next week). However, separate water lines from Mendota were installed for fire-fighting purposes but are no longer used today.
Even with this recent improvement in water use, the impacts of over a century of groundwater pumping have been widespread. Computer models that simulate groundwater flow – and continue to be used and modified by our research project – have estimated that the water table has decreased 10-60 feet in certain parts of the Madison area due to groundwater pumping.
The amount that we pump for drinking water can be hard to visualize due to its diffuse nature, but if you add it all up, it is equal to about half of the average flow in the Yahara River between Lake Mendota and Monona. That’s a lot of water being continuously moved from an aquifer hundreds of feet below ground to the wastewater treatment plant downstream of the Yahara Lakes!
As a result, springs that used to flow around the margins of the Yahara Lakes are now usually just trickles or are completely dry. Groundwater flow to streams, wetlands, and lakes has decreased and negatively impacted these groundwater-dependent ecosystems. These problems are most noticeable during dry periods, when it can be hard to navigate a boat down the Yahara River or near shore on the Yahara Lakes, for example.
While we are not in danger of running out of our municipal water supply from the Mount Simon sandstone aquifer anytime soon, the impacts of groundwater pumping on surface water resources around the Yahara Watershed are concerns for those interested in wetlands, streams, and lake levels. Better water efficiency can certainly lessen the impact, but we will likely never again see the abundance of large springs that used to outline the lakes in the Yahara watershed.
So where does our drinking water go once we use it and send most of it down the drain? That will be the subject of next week’s blog post when we explore the story of wastewater in the Yahara watershed and its connection to the water cycle.