DOM characteristics along the continuum from river to receiving basin: a comparison of freshwater and saline transects

The chemical characteristics of bulk (sterile-filtered) and high molecular-weight dissolved organic matter (HMW DOM) were analyzed for freshwater (St. Louis River, Minnesota to Lake Superior) and saline (Elizabeth River, Virginia to Chesapeake Bay) river-to-receiving basin transects. Dissolved organic carbon concentrations and UV–Visible spectroscopy of bulk DOM demonstrated a reduction in organic carbon, colored DOM and aromatic compounds downstream in both transects. The proportion of DOM recoverable via ultrafiltration as HMW material also decreased downstream in both transects, although there was an offset in recoveries between the transects that may be explained by the effects of ionic strength and/or differences in ultrafiltration technique. The analysis of HMW DOM by Fourier transform infrared spectroscopy illustrated similar trends between transects, with a general shift from aromatic/carboxylic compounds nearshore to aliphatic/carbohydrate materials offshore. The parallel changes observed along saline and freshwater transects imply that similar processes play significant roles in the down-gradient alteration of DOM and that ionic strength or pH changes cause second-order effects.     

Groundwater flow and implications for groundwater contamination north of Prewitt,New Mexico,U.S.A.

In the southern San Juan Basin, New Mexico, strata of Permian and younger age dip gently toward the center of the basin. Most previous investigators believed that recharge to these strata occurred by precipitation on the outcrops and groundwater flowed downdip to the north and northeast. Recent water-level measurements in an undeveloped part of the basin near Prewitt, New Mexico, show that groundwater at shallow depths in alluvium and bedrock flows southward, opposite to the dip direction, and toward a major ephemeral drainage in a strike valley. North of this area, groundwater in deep bedrock aquifers does appear to flow northward. This information suggests that there are two groundwater circulation patterns; a shallow one controlled by topography and a deeper one controlled by geologic structure.Significant amounts of recharge to sandstone aquifers by infiltration through outcrops is unlikely due to the near-vertical exposures on cliffs, the gentle dip of the strata, and small annual precipitation. Numerical model results suggest that recharge to bedrock aquifers may be from downward leakage via aquitards over large areas and leakage from narrow alluvial aquifers in the subcrop area. The recharge mechanism is controlled by the hydraulic conductivity of the strata.As the flow path is controlled by hydraulic conductivity contrasts, geologic structure, and topography, contamination movement from surface impoundments is likely to be difficult to predict without a thorough hydrogeological site investigation.