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No description available.

No description available.

No description available.

No description available.

Groundwater quality in karst systems is difficult to monitor because the extreme heterogeneity within the recharge area and complex subsurface flow network makes flowpaths and travel rates difficult to predict. Understanding how flowpaths vary during storm events is important because water transmitted through conduit flowpaths can travel fast, may come from long distances, and has little filtration of contaminants. The hypothesis tested in this project is that ion ratios in spring discharge will show the timing of changes from diffuse to quick flow depending on storm intensity and antecedent conditions and provide more detail than total ion conductivity. Cedar Run Spring is located in the Cumberland Valley of south-central Pennsylvania. The valley is part of the larger Great Valley Section and is composed of Cambro-Ordovician aged carbonate units, collectively known as the Cumberland Valley Sequence. Initial background monitoring with data loggers and monthly samples indicated that Cedar Run Spring had a conduit component within the flow network. An automated stormwater sampler was installed at the spring and collected twenty-four water samples for major-ion analysis. Storm-intensity conditions ranged from high to low for the four storm events collected. In addition, the antecedent conditions varied from wet to dry. The Mg/Ca ratio characterizes the flowpath through which the water moves. A higher ratio indicates more diffuse flow because slower flow paths are needed to dissolve dolomite (which contains Mg), while a lower ratio indicates more conduit flow because calcite (Ca dominant) dissolves more readily. Hysteresis loops of conductivity versus discharge rotated counterclockwise because conductivity decreased on the rising limb of storm response, followed by an increase on the falling limb for all but the winter storm, which was influenced by road salt. In contrast, hysteresis loops for Mg/Ca versus discharge rotated in a clockwise direction for all but one of the storm events because of an increase in Mg/Ca that indicated a flush of older matrix water. The storm event that did not display in initial increase in Mg/Ca was apparently flushed by a recent previous storm event. Mg/Ca hysteresis for the storm events that were diffuse displayed several sharp increases and decrease in addition to several smaller hysteresis loops in response to multiple slugs of recharge water. These variations were not indicated in overall conductivity. High intensity events displayed a quick switch in flowpaths, as indicated by the increase in Mg/Ca early on the rising limb, and a single hysteresis loop. The rapid change in Mg/Ca suggested that during storm events water was able to enter the karst system through sinkholes, then activated flowpaths with older matrix water. Mg/Ca proved to be better at tracking the variability in flowpaths during storm events than the overall conductivity, because Mg/Ca is directly related to water-rock interactions. / Geology

Time-lapse electrical resistivity tomography (TL-ERT) enables an accurate characterization of the heterogeneity of flow through the unsaturated zone especially when compared to point measurements taken within the same survey area. The most powerful tool for understanding the unsaturated zone is a combination of several techniques. Many models of unsaturated zone flow assume a uniform wetting front even though the existence of preferential flow paths is well-documented in the literature. TL-ERT surveys were collected perpendicular to a stream at the Stroud Water Research Center in Chester County, PA to provide continuous measurement of unsaturated flow during two natural infiltration events. Dielectric sensors were installed along this transect to collect soil moisture data during these events. Additionally, slug tests and infiltrometer tests were collected along the transect to characterize the subsurface at the study site. TL-ERT successfully located sections with preferential flow, and these results were reproducible three months later. Other methods of measuring soil moisture content or infiltration rates were less successful at identifying preferential flow. The rates determined from point measurements often did not match where the TL-ERT identified zones of preferential flow. This comparison reveals that slow-infiltration points can exist within preferential pathways and exemplifies the importance of large-scale measurements in the unsaturated zone. Any scientific study looking at infiltration should consider utilizing TL-ERT to map where preferential flow may be occurring. / Geology

Many grasslands in arid and semi-arid regions are undergoing rapid changes in vegetation, including encroachment of woody plants and invasive grasses, which can alter the rates and patterns of fire and sediment transport in these landscapes. We investigated the spatial distribution of sediments at the scale of vegetated microsites for three years following a prescribed fire using a multiple rare earth element (REE) tracer-based approach in a shrub-grass transition zone in the northern Chihuahuan desert (New Mexico, USA). To this end, we applied REE tracers – holmium, europium, and ytterbium on shrub, grass, and bare microsites, respectively in March 2016. Soil samples were collected from both burned and control (not burned) sites before (March) and after (June) the annual windy season, from 2016 through 2018. Results indicate that although the horizontal mass flux (HMF) of wind-borne sediment increased approximately threefold in the first windy season following the fire, and the HMF of both plots were not significantly different after three windy seasons. Comparing REE concentrations in sediments from both plots over the three years and three annual windy seasons, we observed a post-fire shift in source and sink dynamics of sediments. The tracer analysis of wind-borne sediments indicated that the source of the HMF in the burned site was mostly derived from shrub microsites following the fire, whereas the bare microsites were the major contributors for aeolian sediment in control areas. The shift in sources and sinks, and the spatial homogenization of REEs indicate that the removal of shrub vegetation resulted in sediment redistribution to the bare microsites even three years after the prescribed fire. The findings of this study will improve our understanding of post-fire geomorphic processes at a microsite scale in a grassland ecosystem undergoing land degradation induced by shrub encroachment. / Geology

Urbanization affects in-stream biogeochemical processes that control nutrient export. Attempts to restore urban streams will not be successful unless the biological and physical controls on water quality are thoroughly understood. The objective of this study was to identify the relative influences of tributary dilution, groundwater discharge, and biological processing on nitrate concentrations in an urban stream during high and low flow periods. A wastewater treatment plant (WTP) on Pennypack Creek, an urban stream near Philadelphia, PA, increases nitrate concentrations to a mean of 8.5 mg-l-1 (as N). Concentrations decrease to 5.5 mg-l-1 about 7.5 km downstream. Reaches along this distance were sampled for nitrate concentration and delta-15N at fine spatial intervals to determine the reasons for this decrease. To quantify the effects of dilution, samples were collected from tributaries, groundwater springs, and upstream and downstream of tributaries or groundwater discharge zones identified through terrain analysis and continuous temperature modeling. These methods were also used to identify and sample reaches along which hyporheic flow occurred, where nitrate biological processing is often concentrated. In addition, loggers were installed at closely spaced sites to monitor daily fluctuations in nitrate, dissolved oxygen, and related parameters, which provided further indications of biological processing. Longitudinal sampling revealed decreases in nitrate concentration of 2 and 6.5 mg-l-1 during high and low flow, respectively. During high flow, delta-15N varied from 9.5 to 10.5 per mille downstream of the WTP, while delta-15N varied from 10.14 to 11.06 per mille throughout this reach during low flow. Mixing analysis indicated that groundwater discharge and biological processing both control nitrate concentration during both flow periods. Larger declines in nitrate concentration were observed during low flow than during high flow, and delta-15N fell between biological and groundwater signatures, indicating that both processes were enhanced. Continuous nitrate concentrations displayed distinct diurnal cycles often out-of-phase with dissolved oxygen cycles, indicating autotrophic processing. However, shifts occurred in nitrate cycle timing at a weekly scale wherein daily maximum concentrations were observed as many as 6 hours closer to noon than previously. These shifts were comparable to shifts observed across seasons in other studies, and by the end of the summer, nitrate and dissolved oxygen cycles were in-phase. Furthermore, shifts in nitrate cycles could not be linked to shifts in daily fluctuations of WTP discharge. Longitudinal sampling and continuous monitoring suggest that biological processing is an important control on nitrate concentrations in urban systems, though documenting its signature may be complicated by groundwater discharge and anthropogenic inputs. / Geology