HONR 297Environmental ModelsChapter 2: Ground Water2.3: Typical Quantitative Issues

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Situation 1You are a member of the Board of Health

of your town or city, a part-time volunteer position. You have been called by some acquaintances in town and told that they have noticed the underground gasoline tanks being dug up at a local service station. Since they live about a tenth of a mile from the service station and rely on a drinking-water well on their property, they are wondering whether there is any health risk they should be aware of (see Figure 2.4 in text). What should you do?

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Situation 1Has any other town official been involved in

the situation – for example a town engineer or building inspector?

If not, have someone contact the gas station owner to determine why the tanks are being replaced.

Maybe the tanks were removed due to a “small leak” found in one of the tanks – to be safe brand new tanks are put in.

If the gas station owner says that there is “no problem – we monitored lost product and there wasn’t much missing”, should you believe this and assume everything is fine?

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Situation 1What else could have happened

as a result of the “small leak”?◦Soil near the leaking tank is probably

contaminated.◦Rainwater percolating through the

soil could carry contamination to the water table aquifer.

◦The contamination could flow with the ground water in the aquifer and travel away from the initial leak site.

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Situation 1Quantitative questions:

◦How much gasoline leaked?◦How long has the tank been leaking?◦What is the ground-water flow

direction?◦What is the speed of the ground

water?◦How do we answer these questions?◦If we can answer these questions,

how certain are we of the answers?

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Situation 1 To answer these types of

questions we need to have someone perform underground sampling to figure out the extent of contamination.

To do this, wells are drilled into the ground to a depth that intersects underground aquifers – these wells are called test wells or monitoring wells.

The wells are lined with pipe which has screens (i.e. perforations) at various depths so ground water can flow into the well and be sampled.

Courtesy USGS: http://ga.water.usgs.gov/edu/earthgwwells.html

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Situation 1Issues related to the wells

◦Who do we contact to drill the wells and monitor the wells?

◦How much does it cost to do this work (probably expensive)?

◦What is going on underground – get an idea before drilling any wells?

◦How do we get an idea of what is going on underground?

◦How many wells should be drilled?◦Where should the wells be drilled?◦What if all the wells miss the contamination?

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Situation 1Perform initial sampling with a few wells,

interpret data, and add more wells as needed.Who do we get to interpret the data?Engineers? Geologists? Chemists? Consultants?

Lawyers? Gas station owners? Town officials? Citizens? …

Maybe there are some mathematical calculations that can be used to help with this – if so, what are they?

Note that this type of scenario is happening all over the state of Indiana, or any other state, the country, and the world!

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Situation 2You are a homeowner with a private drinking water

well, but you are located in a town which also has a public water supply that about half the residents are connected to. Because of recent developments in the town, it has been decided to augment the town’s existing water supply by developing an additional well sited as shown in Figure 2.5 in our text. For many years you have been concerned about your location, which is relatively close to a formerly used town landfill, but you have been assured by the town engineer that the landfill has been properly “capped” with clay and topsoil and that the contaminants are not subject to migration. What are some potential issues of concern?

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Situation 2 The new well will draw

water from all directions, including from the river to the west, as well as from the east where your house and the old landfill are located.◦ Could this affect your

well’s water supply?◦ What about the landfill – is

it still stable or could the new municipal well impact the landfill – perhaps cause contamination from the landfill to be pulled towards or under your house?

Courtesy USGS: http://pubs.usgs.gov/gip/gw_ruralhomeowner/

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Situation 2In Situation 1, we are interested in

investigation effects of an occurrence in the past.

In Situation 2, we would be interested in what might happen in the future, so we’d want to find ways to predict future behavior!

Again, mathematical models can help, but which ones?

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General Questions to AddressFor ground water flow in these

situations, the aspects of interest can usually be reduced to the following two questions:

1. How much ground water is flowing through a portion of an aquifer?

2. How fast is the water moving?

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How much ground water is flowing through a portion of an aquifer?

This is important – if we can also estimate how fast a pollutant is seeping into the ground water, then we can estimate the concentration of the pollutant in the aquifer.

This will give us a better idea of the levels of contamination we’d want to look for in samples from monitoring wells.

The key to answering this question in Darcy’s Law.

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How fast is the water moving?

Answering this question will give us an idea of how far the contamination might have spread or how much time we have to try to contain the spread.

The key to this question is the interstitial velocity equation.

We will investigate Darcy’s Law and the interstitial velocity equations in the next two sections!

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ResourcesUSGS:

◦http://ga.water.usgs.gov/edu/earthgwwells.html

◦http://pubs.usgs.gov/gip/gw_ruralhomeowner/