Leachate plume emanating from an old unlined municipal landfill site near the city of Norman, Oklahoma, is discharging into the underlying alluvial aquifer. Subsurface imaging techniques, electrical resistivity tomography and electrical conductivity (EC) logging, were used on the site to detect and map the position of the leachate plume. Anomalous EC zones, delineated with the two methods, correlated with the occurrence of the plume detected by water chemistry analyses from multilevel monitoring wells. Specific conductance, a potential indicator of leachate contamination, ranged from 1861 to 7710 μS/cm in contaminated zones and from 465 to 2180 μS/cm in uncontaminated ground water. Results are in agreement with those from earlier studies that the leachate plume emerges from the landfill along preferential pathways. Additionally, there are indications that the leading edge of the plume has migrated, at least, 200 m away from the landfill in the direction of ground water flow.
Hydroclimatic variability manifests as abrupt shifts, trends, runs, and recurrent cyclical phenomena, collectively referred to as components. In this article, we tested for the presence and magnitude of each component in the Niger Basin (West Africa), using the 0.5 ∘ by 0.5 ∘ gridded annual rainfall and temperature data produced by the Climate Research Unit (CRU) at the University of East Anglia, UK, for the period 1901-2006. The streamflow data was also analysed for different sub-basins of the Niger Basin. Abrupt shifts were tested using a Bayesian and nonparametric approach. Trends were analysed using the Mann-Kendall trend test. Runs were extracted for dry, neutral, and wet conditions, simulated 1000 times based on the skew normal distribution, and used to investigate various run characteristics. Cyclical behaviour was investigated using continuous wavelet analysis. The results show that an abrupt change point occurred in 1969 in the rainfall and streamflow (but not temperature) time series in all subwatersheds of the Niger Basin. The magnitude of the shift in the mean rainfall varied between 16 and 24%. The temperature time series exhibit strong positive trends in all watersheds. Post change point, the rainfall and streamflow time series show positive, though statistically non-significant trends at = 0.1. In contrast, disregarding the change point, all subwatersheds show significant negative trends at = 0.05 and the maximum run lengths are about 4 years long for both dry and wet conditions. Finally, wavelet analysis showed that both rainfall and streamflow in the Niger Basin fluctuate on cycles that are predominantly 2-4 years long, with a few occurrences in the 6-8 years range. Wavelet activity diminished noticeably when a series appeared strongly dominated by trends. The results provide a more comprehensive view of climatic variability than would be obtained from only one or a few components.
GRACE Terrestrial Water Storage (TWS) provides unique and unprecedented perspectives about freshwater availability and change globally. However, GRACE-TWS records are relatively short for long-term hydroclimatic variability studies, dating back to April 2002. In this paper, we made use of Noah Land Surface Model (LSM), and El Niño–Southern Oscillation (ENSO) data in an autoregressive model with exogenous variables (ARX) to reconstruct a 66-year record of TWS for nine major transboundary river basins (TRBs) in Africa. Model performance was evaluated using standard indicators, including the Nash Sutcliffe Efficiency criteria, cumulative density frequency, standardized residuals plots, and model uncertainty bounds. Temporally, the reconstruction results were evaluated for trend, cycles, and mode of variability against ancillary data from the WaterGAP Model (WGHM-TWS) and GPCC-based precipitation anomalies. The temporal pattern reveals good agreement between the reconstructed TWS, WGHM-TWS, and GPCC, (p-value < 0.0001). The reconstructed TWS suggests a significant declining trend across the northern and central TRBs since 1951, while the southern basins show an insignificant trend. The mode of variability analysis indicates short storage periodicity of four to sixteen-month in the northern basins, while strong intra-annual variability in the central and southern basins. The long-term TWS records provide additional support to Africa’s water resources research on hydroclimatic variability and change in shared transboundary water basins.
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