HW-7: 36Cl due: 4/22/08
1) (10p) 36Cl
-
Here is a hypothetical 36Cl data set obtained from a large
confined
aquifer. Consider processes such as evaporation, underground production
of 36Cl and radioactive decay (T1/2 (36Cl)
= 301,000years). Assume that the deposition rate of 36Cl
was
constant over time.
| sample # |
36Cl*1015/Cl |
Cl- (mg/L) |
| 1 |
110 |
15 |
| 2 |
115 |
98 |
| 3 |
108 |
130 |
| 4 |
112 |
200 |
| 5 |
109 |
230 |
| 6 |
50 |
30 |
| 7 |
25 |
80 |
| 8 |
20 |
100 |
| 9 |
15 |
160 |
| 10 |
10 |
190 |
| 11 |
11 |
210 |
-
samples 1-5 are from a recharge area, the remainder are from the
downgradient
confined portion of the aquifer
-
in the table they are sorted by increasing distance from the recharge
area.
-
Plot 36Cl vs. Cl-. The equilibrium concentration
due to underground production was determined as 9 (36Cl*1015/Cl).
-
What kind of processes do play a role and what can you say about the
age
of the water?
2) (10 points) 81Kr
and 36Cl
The Table below shows data for a large aquifer system. Calculate 36Cl
and 81Kr ages against each other. Assume an initial 36Cl/Cl
ratio of the groundwater as 131(±11) × 10−15
and that underground production of 36Cl can be neglected.
Well #
|
Cl−(mg/L) |
36Cl/Cl(× 10−15) |
81Kr/Kr (R/Rair)% |
| 1 |
20 |
76.5 (±3.4) |
52.6 (±6.1) |
2
|
24 |
65 (±3) |
36.5 (±4.2) |
| 3 |
52 |
20.2 (±1.6) |
4.8 (±3.8) |
| 4 |
59 |
72.7 (±2.7) |
30.6 (±3.6) |
5
|
92 |
45.6 (±2.1) |
22.8 (±3.0) |
| 6 |
95 |
12.2 (±5.1) |
12.8 (±3.0) |