Potential of MODFLOW to model hydrological interactions in a Semikarst floodplain of the Ozark Border Forest in the Central United States

Riparian shallow groundwater and nutrient movement is important for aquatic and forest ecosystem health. Understanding stream water (SW)–shallow groundwater (GW) interactions is necessary for proper management of floodplain biodiversity, but it is particularly confounding in underrepresented semikarst hydrogeological systems. The Modular Three-Dimensional Finite-Difference Ground-Water Flow Model (MODFLOW) was used to simulate shallow groundwater flow and nutrient transport processes in a second-growth Ozark border forest for the 2011 water year. MODFLOW provided approximations of hydrologic head that were statistically comparable to observed data (Nash–Sutcliffe = 0.47, r2 = 0.77, root-mean-square error = 0.61 cm, and mean difference = 0.46 cm). Average annual flow estimates indicated that 82% of the reach length was a losing stream, while the remaining 18% was gaining. The reach lost more water to the GW during summer (2405 m3 day-1) relative to fall (2184 m3 day-1), spring (2102 m3 day-1), and winter (1549 m3 day-1) seasons. Model results showed that the shallow aquifer had the highest nitrate loading during the winter season (707 kg day-1). A Particle-Tracking Model for MODFLOW (MODPATH) revealed significant spatial variations between piezometer sites (p = 0.089) in subsurface flow path and travel time, ranging from 213 m and 3.6 yr to 197 m and 11.6 yr. The current study approach is novel with regard to the use of transient flow conditions (as opposed to steady state conditions) in underrepresented semikarst geological systems of the U.S. Midwest. This study emphasizes the significance of semikarst geology in regulating SW–GW hydrologic and nutrient interactions and provides baseline information and modeling predictions that will facilitate future studies and management plans

Stream and shallow groundwater nutrient concentrations in an Ozark forested riparian zone of the central USA

Characterizing spatiotemporal variations in surface water (SW)–shallow groundwater (GW) nutrient concentrations is important to predict stream ecosystem responses to disturbance. Unfortunately, there is a lack of such information from mixed-deciduous semi-karst hydro-geological regions. Nitrate (NO3 -), total phosphorous (P), potassium (K) and ammonium (NH4 +) concentrations were monitored in a case study between an Ozark stream and riparian hardwood forest GW over the 2011 water year in the central USA. Average SW NO3 -, P, K and NH4 + concentrations were 0.53, 0.13, 3.29 and 0.06 mg L-1, respectively. Nine meters from the streambank, average GW NO3 -concentration was 0.01 mg L-1, while P, K and NH4 + concentrations were 0.03, 1.7 and 0.04 mg L-1, respectively. Hyperbolic dilution model results indicated that NO3 - and K exhibited dilution behavior, while NH4 + had a concentration effect and P was hydrologically constant. Observed seasonal NO3 - concentration patterns of winter maxima and summer minima in SW (1.164 and 0.133 mg L-1) and GW (0.019 and 0.011 mg L-1) were supported by previous studies yet exhibited distinct semi-karst characteristics. Results indicate that in addition to relatively low residence time, lower nutrient concentrations in GW (relative to SW) may suggest that shallow GW flow processes are important for vegetation removal and retention of nutrients from streams in semi-karst shallow groundwater systems of the central USA

Genome-wide and fine-resolution association analysis of malaria in West Africa

We report a genome-wide association (GWA) study of severe malaria in The Gambia. The initial GWA scan included 2,500 children genotyped on the Affymetrix 500K GeneChip, and a replication study included 3,400 children. We used this to examine the performance of GWA methods in Africa. We found considerable population stratification, and also that signals of association at known malaria resistance loci were greatly attenuated owing to weak linkage disequilibrium (LD). To investigate possible solutions to the problem of low LD, we focused on the HbS locus, sequencing this region of the genome in 62 Gambian individuals and then using these data to conduct multipoint imputation in the GWA samples. This increased the signal of association, from P = 4 × 10(-7) to P = 4 × 10(-14), with the peak of the signal located precisely at the HbS causal variant. Our findings provide proof of principle that fine-resolution multipoint imputation, based on population-specific sequencing data, can substantially boost authentic GWA signals and enable fine mapping of causal variants in African populations