Subterranean ecosystems - Universidade de Évora

IAH 2007 XXXV Congress - Groundwater and Ecosystems Lisbon, Portugal
depth in the wetland fluctuated between approximately 0.15 and 0.25 m, and the surface area ranged between
3500 and 5500 m 2 . No surface water inflows were observed over this period. Additionally, gas exchange rates
between the wetland and the atmosphere were measured by deliberate injection of SF6, and radon production
within the subsurface was measured on wetland sediment samples.
Figure 1 shows radon activities measured in the wetland in late July 2006. Values range from 0.06 to
1.05 Bq/L (mean 0.32 Bq/L) and appear generally high along the southern and eastern edges. Similar maps were
made in May and October 2006, and showed the same pattern of higher radon activities along the eastern and
southern boundaries. (Mean activities were 0.32 and 0.28 Bq/L in May and October, respectively.) The results
suggest that discharge is occurring primarily along these edges of the wetland. The relative uniformity of radon
activities over time, suggests that groundwater inflow is relatively constant.
Results from gas injection experiments using SF6 gave a gas exchange rate of k = 0.15 ± 0.003 m/day,
and the mean radon production rate within the underlying sediments was estimated to be 1.5 mBq/cm 3 /day.
Based on the measured production rate and an assumed diffusion coefficient, the mean diffusive radon flux from
the sediments was estimated to be F = 11 Bq/m 2 /day, and the radon activity within the wetland that would arise
with no groundwater inflow was estimated to be approximately 0.09 Bq/L. Thus, while diffusion may account
for some of the lower values observed in the western part of the wetland and in the reed bed, the higher values
suggest an additional source of radon, most likely advective flux from groundwater inflow.
Fig. 1. Radon activities (Bq/L) in surface water on 25 July 2006.
Because of the short residence time of radon, it is reasonable to use a steady state approach to estimate
groundwater inflow from the measured radon activity. In the five days prior to 24 May, the mean precipitation
rate was 0.6 mm/day and the mean evaporation rate was 1.0 mm/day. Based on a measured lake area of 3500 m 2 ,
this gives precipitation inflow of 2.1 m 3 /day and evaporative outflow of 3.5 m 3 /day. Assuming cg = 15 Bq/L and
V = 525 m 3 (mean depth of 0.15 m), and that there is no surface inflow at this time, the measured radon activity
of 0.32 Bq/L gives a groundwater inflow rate of 11 m 3 /day. Twenty percent uncertainties in P, E or F have
negligible effect on the estimated groundwater inflow rate. Similar uncertainty in V causes only a 5% uncertainty
in Ig, while a 20% uncertainty in A, k and cg also causes a 20% uncertainty in Ig. In late September – early
October, the lake area was 5500 m 2 , and the measured mean radon activity was 0.28 Bq/L. In the five days
preceding radon measurement, the mean precipitation rate was 0.26 mm/day and the mean evaporation rate was
4.8 mm/day, giving a precipitation inflow of 1.4 m 3 /day and evaporative outflow of 26 m 3 /day. Assuming cg =
15 Bq/L and V = 1160 m 3 (mean depth = 0.21 m), gives Ig = 16 m 3 /day.
Keywords: wetland, radon, isotopes, groundwater, water balance
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