Nitrate Biogeochemistry and Reactive Transport in California Groundwater: LDRD Final Report

Nitrate is the number one drinking water contaminant in the United States. It is pervasive in surface and groundwater systems,and its principal anthropogenic sources have increased dramatically in the last 50 years. In California alone, one third of the public drinking-water wells has been lost since 1988 and nitrate contamination is the most common reason for abandonment. Effective nitrate management in groundwater is complicated by uncertainties related to multiple point and non-point sources, hydrogeologic complexity, geochemical reactivity, and quantification of denitrification processes. In this paper, we review an integrated experimental and simulation-based framework being developed to study the fate of nitrate in a 25 km-long groundwater subbasin south of San Jose, California, a historically agricultural area now undergoing rapid urbanization with increasing demands for groundwater. The modeling approach is driven by a need to integrate new and archival data that support the hypothesis that nitrate fate and transport at the basin scale is intricately related to hydrostratigraphic complexity, variability of flow paths and groundwater residence times, microbial activity, and multiple geochemical reaction mechanisms. This study synthesizes these disparate and multi-scale data into a three-dimensional and highly resolved reactive transport modeling framework

California GAMA Program: Sources and transport of nitrate in shallow groundwater in the Llagas Basin of Santa Clara County, California

A critical component of the State Water Resource Control Board's Groundwater Ambient Monitoring and Assessment (GAMA) Program is to assess the major threats to groundwater resources that supply drinking water to Californians (Belitz et al., 2004). Nitrate is the most pervasive and intractable contaminant in California groundwater and is the focus of special studies under the GAMA program. This report presents results of a study of nitrate contamination in the aquifer beneath the cities of Morgan Hill and Gilroy, CA, in the Llagas Subbasin of Santa Clara County, where high nitrate levels affect several hundred private domestic wells. The main objectives of the study are: (1) to identify the main source(s) of nitrate that issue a flux to the shallow regional aquifer (2) to determine whether denitrification plays a role in the fate of nitrate in the subbasin and (3) to assess the impact that a nitrate management plan implemented by the local water agency has had on the flux of nitrate to the regional aquifer. Analyses of 56 well water samples for major anions and cations, nitrogen and oxygen isotopes of nitrate, dissolved excess nitrogen, tritium and groundwater age, and trace organic compounds, show that synthetic fertilizer is the most likely source of nitrate in highly contaminated wells, and that denitrification is not a significant process in the fate of nitrate in the subbasin except in the area of recycled water application. In addition to identifying contaminant sources, these methods offer a deeper understanding of how the severity and extent of contamination are affected by hydrogeology and groundwater management practices. In the Llagas subbasin, the nitrate problem is amplified in the shallow aquifer because it is highly vulnerable with high vertical recharge rates and rapid lateral transport, but the deeper aquifers are relatively more protected by laterally extensive aquitards. Artificial recharge delivers low-nitrate water and provides a means of long-term remediation. Examination of nitrate concentration in relation to groundwater age indicates that the nitrate management plan has not yet resulted in a decrease in the flux of nitrate to the shallow aquifer in the areas tested

FhuA Barrel-Cork Hybrids Are Active Transporters and Receptors

The crystal structure of Escherichia coli FhuA reveals a β-barrel domain that is closed by a globular cork domain. It has been assumed that the proton motive force of the cytoplasmic membrane through the interaction of the TonB protein with the TonB box of the cork opens the FhuA channel. Yet, deletion of the cork results in an FhuA derivative, FhuAΔ5–160, that still displays TonB-dependent substrate transport and phage receptor activity. To investigate this unexpected finding further, we constructed FhuAΔ5–160 derivatives of FhuA proteins from Salmonella paratyphi B, Salmonella enterica serovar Typhimurium, and Pantoea agglomerans. The FhuAΔ5–160 proteins inserted correctly into the outer membrane, and with the exception of the P. agglomerans protein, transported ferrichrome and albomycin. FhuA hybrids consisting of the β-barrel of one strain and the cork of another strain were active and showed higher TonB-dependent ferrichrome transport rates than the corkless derivatives. Exceptions were the E. coli β-barrel/Salmonella serovar Typhimurium cork hybrid protein and the Salmonella serovar Typhimurium β-barrel/P. agglomerans cork hybrid protein, both of which were less active than the β-barrels alone. Each of the FhuA mutant proteins displayed activity for each of their ligands, except for phage T5, only when coupled to TonB. The hybrid FhuA proteins displayed a similar activity with the E. coli TonB protein as with their cognate TonB proteins. Sensitivity to phages T1, T5, and φ80, rifamycin CGP 4832, and colicin M was determined by the β-barrel, whereas sensitivity to phage ES18 and microcin J25 required both the β-barrel and cork domains. These results demonstrate that the β-barrel domain of FhuA confers activity and specificity and responds to TonB and that the cork domains of various FhuA proteins can be interchanged and contribute to the activities of the FhuA hybrids

Energy-Dependent Conformational Change in the TolA Protein of Escherichia coli Involves Its N-Terminal Domain, TolQ, and TolR

TolQ, TolR, and TolA inner membrane proteins of Escherichia coli are involved in maintaining the stability of the outer membrane. They share homology with the ExbB, ExbD, and TonB proteins, respectively. The last is involved in energy transduction between the inner and the outer membrane, and its conformation has been shown to depend on the presence of the proton motive force (PMF), ExbB, and ExbD. Using limited proteolysis experiments, we investigated whether the conformation of TolA was also affected by the PMF. We found that dissipation of the PMF by uncouplers led to the formation of a proteinase K digestion fragment of TolA not seen when uncouplers are omitted. This fragment was also detected in ΔtolQ, ΔtolR, and tolA(H22P) mutants but, in contrast to the parental strain, was also seen in the absence of uncouplers. We repeated those experiments in outer membrane mutants such as lpp, pal, and Δrfa mutants: the behavior of TolA in lpp mutants was similar to that observed with the parental strain. However, the proteinase K-resistant fragment was never detected in the Δrfa mutant. Altogether, these results suggest that TolA is able to undergo a PMF-dependent change of conformation. This change requires TolQ, TolR, and a functional TolA N-terminal domain. The potential role of this energy-dependent process in the stability of the outer membrane is discussed