Cumulative rainfall collectors – A tool for assessing groundwater recharge

The great majority of Southern African aquifers depend on rainfall for their recharge. The accurate estimation of recharge remains one of the biggest challenges for groundwater investigators. Accurate recharge estimations are needed for proper groundwater management as this governs the estimation of sustainable exploitation. Current estimates of recharge to aquifers range between 0.2 and 3% of annual rainfall for the drier Karoo and Kalahari areas and up to 20% for the winter rainfall region of the Western Cape. Important input for determining recharge is knowledge of the chemical and isotopic composition of rainfall. This paper describes a simple, low-cost and low-maintenance tool, the cumulative rainfall collector (CRC), which provides a cumulative sample of rainfall which is unaffected by evaporation. The instrument is capable of collecting rainfall over periods of up to one year. The crucial aspect is to store sufficient rainfall, to eliminate evaporation by covering the water sample with silicon oil and to reduce interference by birds. CRCs were installed at Struisbaai, the West Coast and the South Coast of the Western Cape Province. CRC data for Struisbaai indicate that recharge to the Table Mountain Group (TMG) Aquifer is 17.4% of mean annual rainfall. The West Coast transect includes two production wellfields. CRC data indicate that recharge is 9.7% to 13.5% for the Bredasdorp Formation of the Langebaan Road Wellfield. At Agter Witzenberg recharge estimations range from 24% to 46% to the Nardouw Formation of the TMG. The South Coast transect encompasses the Klein Karoo Rural Water Supply Scheme and CRC data indicate that recharge to the Peninsula Formation of the TMG is 5%. Water SA Vol.31 (3) 2005: pp.283-29

Identification of sources and infiltration regimes of nitrate in the semi-arid Kalahari: Regional differences and implications for groundwater management

The Kalahari region of southern Africa offers much potential for nitrogen input into its groundwater. High nitrate concentrations in Kalahari groundwater are therefore quite common and are caused by both anthropogenic and natural sources. Forecasting groundwater nitrate concentrations remains challenging. Source identification of nitrate contamination in groundwater is an important first step for groundwater management and quality prediction, and is aided by isotope analysisof nitrate in the water. Comparative data from 3 groundwater study sites in the Kalahari of Botswana and Namibia with widely different characteristics are presented. Two of the sites (Ghanzi and Gobabis) have shallow water tables in fractured quartzite. These aquifers were affected by pollution from cattle wastes (manure) and septic tank outflows resulting in groundwater nitrate levels exceeding 1 000 mg NO3/. and in ƒÂ15N values of between +7 and +20 ñ AIR. Short-term increases of groundwater nitrate concentrations were triggered by exceptional rainfall events occurring every 10-20 years. At the third site (Serowe in Botswana) there is similar land use and land cover, yet the aquifers are deeper, groundwater dates to Late-Pleistocene age and borehole levels do not show a response to present-day high-rainfall episodes. Nitrate levels up to 219 mg/. are found, but lower 15N content (ƒÂ15N of +3 to +8 ñ) indicates a natural origin of the nitrate. In this area pollution nitrate is sufficiently delayed in the vadose zone, reaching the saturated zone much later. The data from all 3 sites suggest that nitrogen management options can only be evaluated once an understanding of sources, processes and flowpatterns has been established

Mid- to Late-Holocene estuarine infilling processes studied by radiocarbon dates, high resolution seismic and biofacies at Vitoria Bay, Espirito Santo, Southeastern Brazil

Vitoria Bay is a 20 km long estuary, morphologically narrow, with a microtidal regime and, as other modern estuaries, was formed during the last post-glacial transgression. The estuarine bed morphology is characterised by a main natural channel limited by tidal flats with developed mangroves. Original radiocarbon dates were obtained for the site. Five radiocarbon ages ranging from 1,010 to 7,240 years BP were obtained from two sedimentary cores, which represent a 5 m thick stratigraphic sequence. The results indicate that, until about 4,000 cal. yrs BP, environmental conditions in Vitoria Bay were still of an open bay, with a free and wide connection with marine waters. During the last 4,000 yrs, the bay has experienced a major regression phase, by becoming more restricted in terms of seawater circulation and probably increasing tidal energy. Three main stratigraphic surfaces were recognised, which limit trangressive, trangressive/highstand and regressive facies. The present channel morphology represents a tidal scouring surface or a tidal diastem, which erodes and truncates regressive facies bedding. Foraminiferal biofacies, which change from marine to brackish and mangrove tidal-flat environments, support the seismic stratigraphic interpretation. Absence of mangrove biofacies at one of the two cores is also an indication of modern tidal ravinement.<br>A Baía de Vitória é um estuário com 20 km de comprimento, morfologicamente estreito, com um regime de micromaré e, como outros estuários modernos, formado durante a última transgressão pós-glacial. A morfologia de fundo do estrato estuarino é caracterizada por um canal natural principal limitado por planícies de maré com manguezais desenvolvidos. Datações de radiocarbono originais foram obtidas para a área. Cinco idades de radiocarbono estendendo-se de 1.010 a 7.240 anos AP foram obtidas através de dois testemunhos de sedimento, representando uma sequência estratigráfica de 5 m de espessura. Os resultados indicam que até aproximadamente 4.000 anos cal. AP, as condições ambientais da Baía de Vitória eram ainda de uma baía aberta, com uma conexão livre e aberta com águas marinhas. Durante os últimos 4.000 anos a baía experimentou uma fase de regressão importante, tornando-se mais restrita em termos de circulação da água do mar e provavelmente aumentando a energia de marés. Três superfícies estratigráficas principais foram reconhecidas, limitando fácies transgressiva, transgressiva/nível de mar alto e regressiva. A morfologia do canal atual representa um diastema de maré, mostrando fácies regressivas truncadas e erodidas. Biofácies de foraminíferos, passando de ambiente marinho para ambiente salobro e de manguezais em planície de maré confirmam a interpretação sismoestratigráfica. A ausência de biofácies de mangue em um dos dois testemunhos é tambémuma indicação de ravinamento de maré atual