The fate of non-limiting solutes and the processes of solute retention in the uMkhuze Wetland system, KwaZulu-Natal, South Africa.

Abstract

Wetlands have long been recognised as enhancing the quality of inflowing waters, particularly regarding the plant macronutrients nitrogen and phosphorus. Any research into non-limiting solutes has largely been of a 'black box' nature, with no insights into mechanism of retention presented. Research in the Okavango Delta, Botswana and preliminary work in the uMkhuze Wetland System, South Africa has identified retention of large amounts of non-limiting solutes within these wetland systems. Chemical sedimentation in the Okavango accounts for 360 000 tonnes per year, while a rough mass balance in the uMkhuze Wetland System suggested retention on a scale of 16 000 tonnes per year. The Yengweni and Totweni Drainage Lines are north-south oriented systems that, together with the uMkhuze River floodplain, were selected to investigate chemical retention in the uMkhuze Wetland System. These drainage lines were once tributaries of the uMkhuze River that have been dammed at their southern ends by alluvial deposition on the uMkhuze River floodplain to form tributary valley lakes. Considering seasonal variations in groundwater levels in combination with conductivity, sites of solute concentration were revealed in the groundwater. The use of chloride as a concentration tracer has indicated that solutes are progressively depleted in the groundwater under the influence of a concentration mechanism, with silicate minerals and calcite attaining saturation. Groundwater chemistry and hydrological factors have highlighted the southern Yengweni and floodplain regions as active sites of solute concentration. In these areas, groundwater elevations are variable, which is mirrored by variation in groundwater chemistry. Although elevated solute concentrations do occur elsewhere, the seasonal variation is less marked. The search for solute sinks in the uMkhuze Wetland System also considered the sediment of the wetland system as a possible sink. Elevated solute concentrations in the groundwater could be linked to the accumulation of minerals in the soil, suggesting precipitation of minerals by saturation under a concentration process. In tho southern Yengweni and floodplain regions, concentrated groundwater bodies were linked to high concentrations of minerals in the soil, including neoformed montmorillonite, and calcite deposits. Other sites of chemical concentration in the groundwater in the northern Yengweni and Totweni Drainage Lines have produced little modification of the reworked marine sands on which the wetland is founded. Processes in the southern Yengweni and floodplain regions are clearly more efficient in removing solutes from the wetland surface water and immobilising them in the soil of the drainage line than is happening in the Totweni and northern Yengweni regions. Transpiration by vegetation seems to be the major factor driving chemical sedimentation in this subtropical system, and as such vegetation in this wetland system is not the passive factor it is often assumed to be. The vegetation of the wetland is itself initiating and perpetuating the retention of chemicals in the system. Hierarchical patch dynamics in combination with the theory of thresholds, derived from geomorphology, is useful for placing chemical sedimentation in wetlands into a spatiotemporal framework that increases understanding of the process, and allows identification of sites where chemical sedimentation is likely to occur in wetlands. There are a number of thresholds that define chemical sedimentation driven by evapotranspiration in the uMkhuze Wetland System, which may be considered at increasing spatiotemporal scales from the microscale of seconds within a limited section of the groundwater, to the macroscale thousands of years at the landscape scale of the wetland system. With increasing scale, the effects of the transformations at each hierarchical level have corresponding increasing influence on the structure and function of the wetland system. The initial threshold is surpassed once concentration products of evapotranspiration are retained to some degree within the wetland system, due to increased residence times of groundwater on modification of the hydrological regime from discharge to recharge. Increased residence times allow the products of seasonal concentration to persist beyond the timescale of seasons. The second threshold is the saturation and precipitation of mineral phases that accumulate within the soil profile. With sufficient accumulation of chemical sediments, the physical properties of the sediment are modified, which reduces the velocity of water flow in the soil (Threshold 3). This has implications for hydrological flows between the surface water and groundwater systems. Threshold 4 is attained once the sediment is modified to such a degree that the chemical sediments act as an aquitard, such that surface water and groundwater may be effectively separated. Extrinsic factors influencing the process of chemical sedimentation, such as the atmospheric water demand, chemical composition and volume of inflowing waters, as well as the nature and density of vegetation, may indicate the potential of a system to sequester chemical sediments but cannot predict their occurrence completely, except maybe at the extremes of semi-arid and arid systems. It is the local and intrinsic factors of hydrological flows, their chemical composition and nature of clastic sediments that will govern residence times of water in the system, and therefore the location, nature and extent of chemical sedimentation. Furthermore, chemical sedimentation driven by evapotranspiration is not limited by sediment type as are adsorption and complexation reactions, which are dependent on the availability of active sites, nor by chemical composition of inflowing waters as this factor simply dictates the suite of minerals produced. Therefore, chemical sedimentation in wetlands is expected in a wide range of settings from temperate to arid, with varied substrates and hydrological regimes. The large-scale removal and retention of solutes within wetland soil has system-wide implications for wetland structure and functioning. The ramifications of chemical evolution of the groundwater and soil extend from influencing the distribution of plants and animals, to geomorphological implications of accumulating chemical sediments, as well as off-site effects including water quality enhancement of water available to downstream systems and users. An understanding of the process of chemical sedimentation in wetlands may inform good management to protect this vital function of wetlands, particularly with increasing development and industrialisation pressures in many areas. Extensive chemical sedimentation has been discovered in both the Okavango Delta, Botswana by Ellery, McCarthy and colleagues and through this study in the uMkhuze Wetland System, with the proposed driving force being transpiration. Vegetation induced chemical sedimentation is a hitherto unknown, although seemingly important component, of chemical processing in tropical and subtropical wetlands, and under certain conditions, even in temperate wetlands. This insight into chemical transformations in wetland systems adds a further dimension to the accepted model of chemical cycling.