In the last twenty years, we've learned a lot about the dynamics of the groundwater underlying Thurston County.
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We know that complex chunks and layers of sediment make up our groundwater aquifers.
Sediments were deposited into our groundwater aquifers by ancient glaciers, rivers, and lakes. These sediments include till (which is dense and compact), sand and gravel (which easily transmits groundwater), and silt (which slowly transmits groundwater).
How Scientists Study Groundwater: Hydrologists specialize in the study of water above and under ground. These scientists have worked with the City of Olympia to help us understand in more detail the distribution of these sediments in our Drinking Water Protection Areas.
Scientists use well driller records to piece together a picture of what underground sediments are like. Well drillers are required by the Washington Department of Ecology to submit a log of the types of sediments they find at different depths every time they drill a new well.
With this information, especially for the deeper wells, hydrologists can create a cross section map of what the sediments look like. A cross section is a side view of a cut into the earth; it looks similar to a road cut or a cliff face.
Hydrologists also use computer models to mathematically recreate the inflows (recharge) and outputs (discharge) of aquifers. These computer models allow engineers and planners to understand in more detail current groundwater conditions, how quickly contaminants would travel towards our drinking water sources, and how pumping additional groundwater might impact water levels in the aquifer, and surface water bodies (rivers, lakes, and streams).
Olympia is using this type of sophisticated computer model in planning for a new drinking water well above McAllister Springs.
Groundwater Computer Models: Hydrologists use several different pieces of information to simulate the dynamics of the water cycle. Here are some:
- Geologic information from well logs, groundwater levels, and the rate at which water travels underground.
- Aquifer characteristics - permeability, thickness, location, extent, and boundaries.
- Volume of water leaving streams.
- Volume of water leaving springs.
- Water levels in lakes.
- Recharge characteristics - precipitation, runoff, infiltration of water into the ground.
- Quantity of water pumped under different scenarios.
What We've Learned about Groundwater: In all of our Drinking Water Protection Areas, we have a good understanding of the underlying geology. We also know how long it takes water, at different locations on the land surface, to flow underground to our water sources.
In the McAllister Drinking Water Protection Area, we know that groundwater levels and daily rainfall mimic each other very closely. Using the computer model, we are able to predict what the groundwater levels were historically, even at a time when we only had data on rainfall. There is a delay between when rain falls, and when aquifer levels change. So, we can predict future groundwater levels and compare those to the actual levels we see during pumping.
We also have a water budget for the McAllister area which estimates the amount of water flowing into and out of the area. Water flowing in comes from rainfall, and water leaving the area goes into streams, rivers, Puget Sound, well pumping, and evaporation.
Need More Information? Contact the Public Works office.
Address: 924 7th Ave. SE, Suite A
Download a printer friendly (PDF) copy of directions to our office Hours: Monday - Friday from 8:00 a.m. to 5:00 p.m.
Phone: (360) 753-8588
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