Problem 3 130 points]. One of the most detailed groundwater tracer experiment si

Question

Problem 3 130 points]. One of the most detailed groundwater tracer experiment sites in the U.S. is located on the Columbus Air Force Base in Mississippi. Over a period of 48 hours in October 1986, a total of 10.07m of bromide solution at a concentration of 2500 mg/L was injected below the surface of a grassy field into a shallow sandy aquifer (porosity 0.41 + 0.03, median grain size 1.0+0.3 mm, horizontal hydraulic conductivity 1.2 ± O 7 m/d, vertical aquifer thickness 10 m) with a planar head gradient. The bromide was released from five injection wells spaced 1 m apart perpe centered at location X-0, -0 on the map. Gver the next two years, the concentration of bromide was tracked using an array of closely spaced wells, most of which allowed sampling at 20 to 30 different vertical locations. Please submit your spreadsheet electronically and on paper. 1 50 100 The table (available as a CSV file on myCourses) provides measurements of ground elevation relative to mean sea level, Fe Muldlevel sampling well network (tracer injection point at height of top of casing above ground level ("casing heighf"). distance of water table below top of casing ("depth to water table"), and distance from top of casing to well sampling port ("sampling depth) at the eight labeled wells. Measurements represent the average of six different measurements during the months following tracer release coordinates eo.?. Well A Well B Well C WellD Well E Well F Well G Well H -36 29 Lateral location, X() Longitudinal location, Y() Ground elevation, god( 12838 12831 128.14 127.88 12821 128.06 12782 127.73 Casing height, AzToc () 243 85 0.34 0.19 055 0.59 0.470.310.450.49 Depth to water table, 4tor (n) 2.982.16 19 126 294 203 148 1.01 Sampling depth A (n)3 5.18 4.97 5.15 4.92 5.15 4.83 4.734.68 a) For each well, calculate the (i) elevation of the well top of casing, () water table elevation, (ii) sampling port elevation, (iv) elevation head of the water sampled for bromide, and (v) pressure head of the water sampled for bromide. Show all calculations (including equations with variables) for Well A. You do not need to show all your calculations for the rest of the wells; instead, display results for all wells (Wells A-H) in a table. b) Assuming a planar water table, calculate the head gradient in the aquifer with uncertainty Explain in words/variables how you performed this calculation; just showing a spreadsheet is not sufficient. (Hint: use LINEST) c) Draw an arrow on the above map showing your prediction for the direction of bromide movement within the aquifer. Estimate the velocity of bromide advection with uncertainty d) Assuming cloud spreading in the longitudinal direction was controlled by mechanical dispersion, how long should the depth-averaged tracer cloud have been after 503 days? Draw a shape oa the above map that yoru would expect to encircle 95% ofthe mass in the cloud at 503 days after release Assume lateral and vetcal diffusion coefficients of 0.004 m ld. e) What was the expected peak concentration within the depth-averaged tracer cloud after 503 days, before the tracer cloud had spread to touch horizontal boundari

Explanation / Answer

i) elevation of the well top of casing = zground + zTOC

well A: 128.38 + 0.34 = 128.72 m

well B: 128.5 m

well C: 128.69 m

well D: 128.47 m

well E: 128.68 m

well F: 128.37 m

well G: 128.27 m

well H: 128.22 m

ii) water table elevation = zground - zwt

well A: 125.4 m

well B: 126.15 m

well C: 126.24 m

well D: 126.62 m

well E: 125.27 m

well F:126.03 m

well G: 126.34 m

well H: 126.72 m

iii) sampling port elevation = zwelltop - zsample

well A: 123.54 m

well B: 123.53 m

well C: 123.54 m

well D: 123.55 m

well E: 123.53 m

well F: 123.54 m

well G: 123.54 m

well H: 123.54 m

iv) elevation head of water sampled =  sampling port elevation = zwelltop - zsample

well A: 123.54 m

well B: 123.53 m

well C: 123.54 m

well D: 123.55 m

well E: 123.53 m

well F: 123.54 m

well G: 123.54 m

well H: 123.54 m

v) pressure head of water sampled = water table elevation - elevation head

well A: 1.86 m

well B: 2.62 m

well C: 2.7 m

well D: 3.07 m

well E: 1.72 m

well F: 2.49 m

well G: 2.8 m

well H: 3.18 m

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