BTFSOC
Assignment 8 – Estimating GW contamination
In an area near Aarhus in Denmark a potentiometric map has been constructed based on
measurements of water levels in some wells in the area. The wells are all placed in the same
unconfined aquifer of meltwater sand layers.
Water levels in
meters relative
to sea-lavel
N
Approx scale:
0
1 km
Blues numbers: potentiometric level of GW [masl]. Yellow numbers are well ID number.
In the northern end of the area there is a lake. The water level in the lake is approx. 0.5 m above
sea-level (a small river runs from the lake to the nearby sea).
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Questions:
1) What could be a possible explanation to the fact that the level of the ground water in the
northern end of the area (e.g. in well 89.1351) is below sea level? Does groundwater water
flow into the lake here, or does water flow from the lake to the ground?
2) Assume that we have an oil contamination 500 m straight north of well 89.1263 (in the
south west end of the area). Draw in the map a likely flow-line for a water particle that
infiltrate this soil from the surface near this well (as net precipitation), i.e. water infiltrating
the ground here in which direction does it flow? Is there a risk that this water could
contaminate drinking water intake from any of the wells in the area?
3) If the soil in the area around the contamination is sand with a hydraulic conductivity of
2·10-5 m/s, what is the Darcy velocity of the groundwater? Assume an effective porosity of
the soil and calculate the real velocity of the groundwater? How far will the water-particle
infiltrating here travel in one year?
4) The contaminated are is estimated to be a circle with a diameter of 6 m. in this area the soil
water has an average contamination of benzene of 20 mg/l. Use JAGG to calculate an
estimated value of the concentration of benzene in the groundwater below the contaminated
site C1 (step Ia calculation), assume a net precipitation of 200 mm/year . Compare with the
groundwater quality criteria. (write down any assumptions about the situation you may have
to make in order to get all data JAGG require)
5) Use JAGG to calculate C2 and C3 (step IIa and III respectively). Again write down
assumptions you may have to make. Use default values from JAGG for sand for effective
porosity, organic matter content and for degradation constant under oxic conditions.
6) How long time do you think it would take the oil contamination before we could see impact
on the water quality in the nearest downstream well? Which (unknown) elements in the case
could influence your risk assessment?
7) Redo calculation step IIa and III with
a. Hydraulic conductivity of 10-5 m/s - what is the effect?
b. fOC in the aquifer of 0,003
c. Net precipitation of 300 mm
Which has the bigger effect on the GW concentration 1 year downstream (and how
far away is this?)
8) Which initiatives would you suggest for further investigation or remediation?
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