Hydrological and chemical connectivity dynamics in a groundwater‐dependent ecosystem impacted by acid sulfate soils

Abstract

[1] Groundwater-dependent ecosystems (GDEs) in arid and semiarid environments play significant ecological roles, and, yet in many parts of the world, these ecosystems have been drained for agricultural use. In wetlands containing acid sulfate soils, the altered hydrology may trigger acidification and subsequent trace metal release. Quantifying shifts in hydrological regime and connectivity dynamics across wetlands is critical for understanding the resilience of these GDEs to anthropogenic impacts. Seasonal water balances for a wetland severely impacted by drainage and acidification were combined with laboratory geochemical data and field observations to develop a conceptual model describing hydrological connectivity across the wetland. The data indicated that, with the onset of the dry season, the superficial aquifer was lowered, exposing sulfides that oxidized to form sulfuric acid and dissolving metal salts. The following dry season enhanced capillary action causing upwelling of oxidized products to the surface where evaporative precipitation created acidity scalds. Subsequent winter rainfall and infiltration caused groundwater levels to rise, intersect with the ground surface, and form disconnected acidic pools. As the wet season progressed, connectivity was established between the pools, resulting in metal-rich acid discharge from the wetland. The degree of acid fluxes and metal release was controlled by the physicochemical characteristics of the soils, its exposure to the seasonally variable wetland hydrology, antecedent hydrological conditions, hydrological connectivity (both vertical and horizontal), and the resulting biogeochemical conditions.

Dissolved organic carbon characteristics in an acidified groundwater-dependent ecosystem

“…As at Bore #6, there was a lag of almost 6 months between winter rainfall ceasing in October and the maximum water levels observed in April. Similarly there was a 6-month lag between maximum evaporative demand in May, and the minimum water levels observed in October (Nath et al 2013). Over the 3-year sampling period, lower pH conditions were observed at Bore #3 when water levels dropped and higher pH conditions were observed when water levels came closer to the surface.…”

“…Nath et al (2013) showed that seasonal variability in levels of both Minninup Lake ground and surface waters interacted with spatial variability in ground levels to create seasonally and spatially variable ponding across the wetland. The difference between summer and winter groundwater levels ranged from À0.5 to 3.0 m; these spatial differences in depth to groundwater affect the environmental conditions experienced at different locations.…”

“…The difference between summer and winter groundwater levels ranged from À0.5 to 3.0 m; these spatial differences in depth to groundwater affect the environmental conditions experienced at different locations. The horizontal connectivity of the ponded areas and the vertical connectivity between ponded areas and groundwater determined the discharge to drains and the wetland hydrology, including its water balance (Nath et al 2013).…”

“…The elevations determined from LiDAR DEM are shown. Surface run-off and groundwater discharge to the major drain (flowing north to south) and flow to another wetland whose levels are controlled by flood gates to the Indian Ocean Nath et al (2013). provide details of how standing water within the wetland varies with time and space.…”

Groundwater Dependent Ecosystems: Classification, Identification Techniques and Threats

“…Changes in water table depth, coupled with other environmental factors, can also result in groundwater contamination. For example, lowering a water table beneath acid sulphate soils leads to oxidation of pyrite and subsequent acidification of the shallow aquifer (Ritsema et al 1992;Nath et al 2013). …”