Well, Well, Well! How Deep is the Water Table?

[Intro Music: Scott Pemberton Trio, Little Bobby Cobalt]

[Segment #1: Introduction to Portland Groundwater]

[Steven Sobieszczyk] Hello and welcome! This is the USGS Oregon Science Podcast for Tuesday, July 27, 2010. I'm Steven Sobieszczyk.

For today';s episode we’re back to getting our feet wet as we focus on another water-related topic here in Oregon. Finally, after nine podcast episodes we're going to bring you our first look at subject of groundwater...definitely, one of our favorite topics. In the next segment, I'll play an interview I recorded a few weeks ago with USGS hydrologist Daniel Snyder. Dan recently completed work examining the depth of the water table here in Portland. Information about the groundwater table in Portland can be found in the USGS Report SIR 2008-5059. However, if you prefer a more interactive experience, you can also access site-specific water table depths at the project website, which is linked on our podcast homepage and in our show transcripts. The new Google-based, interactive map shows the estimated regional groundwater depth, surface elevations, and a relative error or uncertainty for these estimates. It's a pretty nifty application; so please, take some time, play around, check it out.

In a minute we'll be back with that interview I promised with Dan Snyder. Dan will explain what affects water table depth, why groundwater is important, and how you go about estimating something you can't really see, but know is actually below your feet. Please, stick around, we'll be right back.

[Segment #2: Interview with Dan Snyder]

[Steven Sobieszczyk] Welcome back. It's now time for today's interview. I'm pleased to be joined by USGS hydrologist Dan Snyder. A little over a year ago, Dan published a report describing the estimated depth to groundwater and the configuration of the water table here in Portland, Oregon. This was an important study because it was the first time someone actually mapped, in detail, the location and depths of the water table for the entire city. The project was a wonderful example of cooperation between the USGS and other state and local agencies, including: the City of Portland, the City of Gresham, Clackamas County's Water Environment Services, and Multnomah County.

In early May 2010, this report got a whole new life as a web-based tool. Residents in Portland can now browse the website and access groundwater information through an interactive Google map. Simply point and click on a location or type in your address and you can find out a little bit more about surface elevations and groundwater depths anywhere in the city. Pretty cool stuff.

Well, Dan, thanks for joining us today. [Morning Steve]

First off, how did this project come about?

[Dan Snyder] The City of Portland approached the USGS and asked if we could provide depth to water in the Portland area for their installation of a new type of storm drain called a dry well. This is a hole underneath existing storm drains that routes water to the groundwater system. And there is a requirement to have a specified separation between the bottom of this feature, this dry well, and the water table.

[Steven Sobieszczyk] Why is it important to know the location and depth of the water table here in Portland?

[Dan Snyder] The depth to water is one of the most frequent questions we receive at the USGS. It's important information in terms of construction for buildings or roads to find out what the design might be where they encounter water. It's important for drilling wells for determining how far the well must be drilled in order to reach water. And finally, it's important in order to protect the groundwater resource from preventing surface water and contaminants contained in surface water from entering the groundwater system.

[Steven Sobieszczyk] Assuming that this isn’t just a theoretical model, how did you determine the actual depth to the water table? Where did you get the groundwater measurements you used in your model?

[Dan Snyder] We used information from about 600 wells measured by the USGS over the last several decades. Many of these wells have measurements that have taken place over numerous years, and we use the average of those measurements to come up with a value for each location.

[Steven Sobieszczyk] What method did you use to actually estimate the depth to the water table between the known well locations?

[Dan Snyder] Well, the method of analysis we used was kriging. It's a mathematical method of interpolating between known observations of water levels. We chose kriging because its considered one of the most robust methods of interpolating between points. It gives greater emphasis to observations that are nearer the point of interest than further away.

[Steven Sobieszczyk] There's inherently going to be changes to the water table as seasons change. More in the winter and less in the summer depending on how much water is available, what type of conditions does your model represent and how do you account for this seasonal variability?

[Dan Snyder] The map of depth to water that we created represents the average groundwater conditions. However, seasonal changes in the amount of precipitation that occurs will cause the water levels to fluctuate. The water level fluctuation is also a function of the type of rocks. Some rocks have greater porosity, which is more ability to store water in the pore spaces between the grains of soil or rock. The more space available, the less water level change generally takes place.

[Steven Sobieszczyk] Now that you can visualize the entire Portland area water table, are there any areas that stand out as either being extremely shallow or significantly deep? And, what factors likely contribute to such a disparity in the water levels?

[Dan Snyder] The relationship between groundwater levels and the geologic history and topography of the area is what really seems to control the groundwater levels. It's long been known that the water table is a subdued replica of the land surface. Areas of higher elevation generally have deeper depths to water. Low-lying areas, or areas in valleys, generally have shallower depths to water. In Portland, the presence of the volcanic hills, such as Rocky Butte, Mount Tabor, and Powell Butte, generally have greater depths to water because of the relatively low permeability of the rocks. In much of the area extending eastward from the Willamette River, through northeastern and southeastern Portland, to the mouth of the Sandy River there are a series of terraces. These terraces were created by a series of tremendous floods that came out of the Columbia River Gorge between 12,000 and 15,000 years ago. Now these floods deposited large quantities of silt, sand, gravel, and boulders and they are very permeable. They are so permeable that most of the precipitation that falls in this area infiltrates through the soil. As a result, there are very few streams in this area, and the water table there is relatively flat. In this area, the depth to water is primarily the function of the thickness of the overlying sediments.

[Steven Sobieszczyk] Well, Dan, that seems like a very interesting story and I'm hoping that some of our listeners will go check out the website and get an idea of how their house or how their parent’s house water table will vary between them. Thanks for joining me today.

[Dan Snyder] Thank you Steve, I hope folks will find the information both interesting and useful.

[Steven Sobieszczyk] Well, that’s all we have for today’s show. Thanks for listening. If you want to check out any of the links in our transcripts, you can find them at our website: or.usgs.gov/podcasts. If feel you want more frequent local USGS news and insight, you can find us on Twitter at "USGS_OR." As always, if you have any questions, comments, or even complaints about the USGS Oregon Science Podcast, please feel free to email us at oregonpodcast@usgs.gov. Thank you for listening. To hear more about other research the USGS is doing around the country, pick any one of our other USGS social media outlets at: usgs.gov/socialmedia. There you can listen to other USGS podcasts, as well as find links to USGS on Twitter, YouTube, Facebook, and Flickr.

Until next time. I'm Steven Sobieszczyk.

This podcast is a product of the U.S. Geological Survey, Department of the Interior.

[Outro Music: Scott Pemberton Trio, Little Bobby Cobalt]