Water quality and geochemistry of the mountain fynbos ecosystem in the vicinity of Citrusdal, South Africa

The water chemistry along the path of the hydrologic cycle gives clues to the complex interactions among water and the bedrock, soil, vegetation and atmosphere. This study gives a first-order estimation of the chemical composition of the recharge, discharge, and ground waters, along with the bedrock, soil, and vegetation of the Olifants River Valley around Citrusdal, South Africa. The valley occurs in a synclinal fold with the main aquifers, the Table Mountain Group (TMG) sandstones of the Peninsula Formation and the Nardouw Subgroup, folded beneath the central valley. The Peninsula aquifer is recharged in the east towards the Cedarberg Mountains and discharged at up to 43° C in the west. The headwater catchments support mountain fynbos vegetation communities, part of the Cape Floral Kingdom, which is globally significant as one of 6 floral kingdoms in the world and a biodiversity hotspot. Groundwater data for this study comes from two boreholes, one cold spring, and one warm spring. Ten surface water samples were taken to study discharge, and 14 rainwater samples for recharge (3 from Citrusdal, 11 from Cape Town). Alkalinity and acidity titrations were performed in the field to complement pH values in characterizing the acid-base status of the waters. Major ions were determined by ion chromatography, and trace elements by ICP-MS. The recharge (pH 4.8-5.8) carries roughly a seawater signature, with some deviation from rainout and washout of wind-blown dust. Rainwater composition in the study area is similar to that sampled within 5 km of the coast in Cape Town, located 170 km south of the study area. Discharge is acidic in the study area (pH 4.9-5.8) and varies from clear to light brown (DOC <3 mg/l). TMG discharge is commonly acidic, particularly from the Peninsula Formation, where there is minimal buffering from weathering of the quartz arenite sandstones. In addition, organic acids tend to pass through with the discharge in these clay-poor sandy soils (<7 wt% clay). TMG ground waters studied have low ionic strength (0.2-1 mM) and electrical conductivity (2.5-12 mS/m). The low pH helps to keep trace metals in solution, particularly iron and manganese, which tend to precipitate in the presence of oxygen near the surface. The presence of metals in solution contributes to the ability of the ground waters studied to buffer the addition of hydroxyl ion (Base Neutralizing Capacity) in the acidity titration. The Acid Neutralizing Capacity, or the ability to buffer acid, increases downstream along the main valley as agricultural runoff increases. The size of the major elemental reservoirs and the fluxes among them are approximated from chemical analyses of the bedrock, soil, vegetation, and waters of the ecosystem. These estimates are used in a simplified box model to discuss the element dynamics within the fynbos ecosystem of a headwater catchment underlain by the Peninsula Formation. The data support the idea that soil and vegetation act as a filter, selectively retaining elements from the water, particularly K and Mn, but also including Ca, Mg, Fe, and Al. The properties of the ecosystem that allow for this retention of elements could be disrupted by human activities.