Ebb and Flow: Feeling Like a Yo-yo
By Sean Sheldrake
My previous blog posts have featured how EPA diving scientists support cleanups in the nation’s waterways. In this post, I talk about how our divers study the connection between groundwater and our waterways to support EPA cleanups. (Hint: it’s not a one way street!) Understanding which way groundwater is flowing is critical to implementing a successful cleanup—and protecting our nation’s waterways and oceans.
Groundwater: coming or going?
We all know storm drains connect to our waterways, but how about groundwater? In a given stretch of a stream, lake, river, or the seafloor, groundwater may be feeding the waterway—or the opposite—that stream, lake, river, or ocean could be losing water into groundwater in that location. The direction of the flow can change by the hour, day, season, and conditions (such as drought)—a reality of the interconnectedness of water in the environment.
Gaining or losing? Mapping out the flow to get the cleanup right
Determining whether a river loses or gains water from the ground is a big deal when devising the best course of action to take during cleanup activities, as we need to follow the contaminated water to wherever it goes. With this information, we can decide on important details, such as where to install caps in a riverbed to stop the flow of contaminated discharge, or how many and how fast pumping wells should be employed to move contaminated groundwater to a treatment plant.
Making such a determination is an ongoing process. For example, in an estuarine river (the part of a river that is near the sea), this may take a lot of monitoring locations over time to know we’re choosing the right kind of cleanup. A lot of factors also need to be considered, including the location, direction, and volume of local ship traffic. EPA divers often must check various locations in the sediment near an active cleanup to determine where groundwater is discharging into the river—and vice versa.
Low-tech goes underwater
At some sites near marine environments, we use conductivity mapping to determine where groundwater discharge is occurring. Because salt water conducts electricity better than fresh water, we tow an array of electrical cables that measure electrical fields to produce a map of where fresh ground water is discharging into salt water. In other sites, we can use a more low-tech approach.
The photo above shows one such technique. Here, we use a five-gallon bucket, cut in half, stuck into the lake bottom. We then outfitted it with a sampling bag filled half way with water. We use this simple device to determine the direction of the water flow by noting what happens to the sampling bag. If it begins to empty we know the direction of water is OUT of the lake (and bag) and into the ground, and if it fills up, we know the water is flowing in the other direction: from the ground into the river. We can also seal the bag and bring it to our lab for analysis, getting and even better understanding of the rate of contaminant discharge into a lake or stream. Over time, divers come back to visit the site to map the wonderful complexity of water’s connections. The map allows us to understand the movement of historical pollution and to determine how to best conduct a clean up.
Read more about the latest in EPA scientific diving at facebook.com/EPADivers.
About the author: Sean Sheldrake is part of the Seattle EPA Dive unit and is also a project manager working on the Portland Harbor cleanup in Oregon. Sean Sheldrake serves on the EPA diving safety board, responsible for setting EPA diving policy requirements.
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