[1] In humid regions a strong coupling between surface water features and groundwater systems may exist. In these environments the exchange of water and solute depends primarily on the hydraulic gradient between the reservoirs. We hypothesize that daily changes in river stage associated with anthropogenic water releases (such as those from a hydroelectric dam) cause anomalous mixing in the near-stream environment by creating large hydraulic head gradients between the stream and adjacent aquifer. We present field observations of hydraulic gradient reversals in a shallow aquifer. Important physical processes observed in the field are explicitly reproduced in a physically based two-dimensional numerical model of groundwater flow coupled to a simplistic surface water boundary condition. Mass transport simulations of a conservative solute introduced into the surface water are performed and examined relative to a stream condition without stage fluctuations. Simulations of 20 d for both fluctuating river stage and fixed high river stage show that more mass is introduced into the aquifer from the stream in the oscillating case even though the net water flux is zero. Enhanced transport by mechanical dispersion leads to mass being driven away from the hydraulic zone of influence of the river. The modification of local hydraulic gradients is likely to be important for understanding dissolved mass transport in near-stream aquifer environments and can influence exchange zone processes under conditions of high-frequency stream stage changes.
Objective
To identify factors associated with the development of surgical site infection among adult patients undergoing renal transplantation.
Design
A retrospective cohort study
Setting
An urban tertiary care center in Baltimore, MD with a well-established renal transplantation program that performs approximately 200–250 renal transplant procedures annually.
Results
441 adult patients underwent renal transplantation from January 1st, 2010 and December 31st, 2011. Fifteen percent (66/441) of cohort patients developed an SSI; 47% (31/66) of these were superficial-incisional and 53% (35/66) were deep-incisional or organ-space. The average BMI among cohort patients was 29.7 and 42% (184/441) were obese (BMI > 30). Patients who developed SSI had a greater mean BMI (31.7 vs 29.4, p =0.004) and were more likely to have a history of peripheral vascular disease, rheumatologic disease, and narcotic abuse. History of cerebral vascular disease was protective. Multivariate analysis showed BMI (Odds Ratio (OR) 1.06; 95% Confidence Interval (CI): 1.02 to 1.11) and past history of narcotic use/abuse (OR 4.86, 95% CI: 1.24 to 19.12) to be significantly associated with development of SSI after controlling for National Healthcare Surveillance Network (NHSN) Score and presence of cerebrovascular, peripheral vascular and rheumatologic disease.
Conclusions
We identified higher BMI as a risk factor for the development of SSI following renal transplantation. Of note, neither aggregate comorbidity scores nor NHSN risk index were associated with SSI in this population. Additional risk adjustment measures and research in this area is needed to compare SSIs across transplant centers.
Spatial and temporal patterns in low streamflows were investigated for 183 streamgages located in the Chesapeake Bay Watershed for the period 1939–2013. Metrics that represent different aspects of the frequency and magnitude of low streamflows were examined for trends: (1) the annual time series of seven‐day average minimum streamflow, (2) the scaled average deficit at or below the 2% mean daily streamflow value relative to a base period between 1939 and 1970, and (3) the annual number of days below the 2% threshold. Trends in these statistics showed spatial cohesion, with increasing low streamflow volume at streamgages located in the northern uplands of the Chesapeake Bay Watershed and decreasing low streamflow volume at streamgages in the southern part of the watershed. For a small subset of streamgages (12%), conflicting trend patterns were observed between the seven‐day average minimum streamflow and the below‐threshold time series and these appear to be related to upstream diversions or the influence of reservoir‐influenced streamflows in their contributing watersheds. Using multivariate classification techniques, mean annual precipitation and fraction of precipitation falling as snow appear to be broad controls of increasing and decreasing low‐flow trends. Further investigation of seasonal precipitation patterns shows summer rainfall patterns, driven by the Atlantic Multidecadal Oscillation, as the main driver of low streamflows in the Chesapeake Bay Watershed.
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