Sedimentary Bedrock Research Articles

Interaction between runoff – bedrock groundwater in a steep headwater catchment underlain by sedimentary bedrock fractured by gravitational deformation

AbstractRecent studies have suggested the importance of the bedrock groundwater (BG) contribution in storm runoff in headwater catchments. However, few such studies have been conducted, and the study of different types of bedrock conditions is still ongoing. The role of BG in storm runoff is still poorly understood, particularly in headwater catchments underlain by relatively deep fractured bedrock. This study aims to clarify this role using hydrometric and hydrochemical observations of BG via boreholes and catchment discharge. The responses of the BG to rainfall are demonstrated to be fast and independent of the sediment cover. The BG exhibits different responses and flow paths that are largely controlled by the bedrock fracture system. The storm runoff in the studied catchment is characterized by rapid discharge response generally followed by a delayed discharge response. The peak of the delayed discharge is much faster than that observed in previous studies, and it is well correlated with the BG levels. A hydrograph separation was performed for two storm events using three end members: rainfall, shallow BG and deep BG. The results demonstrate that the delayed discharge is primarily composed of deep BG. Moreover, a significant contribution of shallow BG is observed during large precipitation events. Although we observed no physical evidence of direct contributions of BG in the catchment, the calculations presented in this study demonstrate that the BG controls the hydrological and hydrogeological response of the catchment to rainfall events. Copyright © 2015 John Wiley & Sons, Ltd.

Hydrochemical evolution within a large alluvial groundwater resource overlying a shallow coal seam gas reservoir

A combination of multivariate statistical techniques, simple hydrochemical mixing models and inverse geochemical modelling was used to investigate the major hydrochemical evolutionary pathways of a large alluvial aquifer, the upper Condamine River alluvium, south-east Queensland, Australia. Hydrochemical similarities between alluvium and sedimentary bedrock groundwater imply some mixing between alluvial and sedimentary bedrock aquifers, but spatial assessment showed that this was localised around outcrops of sedimentary bedrock in upstream areas. Within the alluvium, a distinct shift towards a low salinity Na–HCO3 water type and a brackish Na–HCO3–Cl water type was obvious in two separate locations. Both of these water types are unique to the alluvium, and inverse modelling shows that they can evolve via a combination of in situ alluvial processes, including diffuse recharge of rainfall or river water or the evolution of basalt-derived groundwater via gypsum dissolution plagioclase weathering, cation exchange and some carbonate precipitation/dissolution. The evolution of these water types is potentially influenced by overlying sodic alkaline soils, and often is associated with a source of sulfate. Evapotranspiration is the dominant salinization process in the alluvium and increases in calcium cations during salinization indicate that brackish Na–HCO3–Cl groundwater in the underlying Walloon Coal Measures are unlikely to have a major influence on salinization in the alluvium. The most saline water types observed were endemic to shallow zones of the alluvium where evapotranspiration is likely. Results demonstrate that a combination of multivariate statistics and inverse geochemical modelling can be successfully used to delineate hydrochemical pathways in complex hydrogeological settings where a range of environmental and anthropogenic factors may be influencing the evolution of water types with similar hydrochemical compositions.

Simulation of groundwater flow system in alluvium and fractured weathered bedrock zone: Sand Lick watershed, Boone County, West Virginia, USA

The Appalachian Plateau is characterized by relatively flat-laying but intensely eroded bedrock, comprising cyclical sequences of Pennsylvanian age sedimentary bedrock dominated by sandstone, siltstone, shale, coal, claystone and occasionally limestone. Fractured/weathered sandstone is potentially the main groundwater transmitting zone. Conventional field or laboratory methods of estimating the hydraulic properties in selected points within such fractured and weathered bedrock groundwater transmitting zones are seldom representative or feasible because of their heterogeneity, anisotropy and irregular aquifer geometry. Therefore, the objectives of our study is to determine the “bulk” hydraulic properties of a weathered/fractured zone in a valley located within the Appalachian Plateau Geomorphic Province as well as recharge and evapotranspiration rates by inverse numerical simulation of groundwater flow.

Evaluating local-scale anisotropy and heterogeneity along a fractured sedimentary bedrock river using EM azimuthal resistivity and ground-penetrating radar

Fractured sedimentary bedrock rivers exhibit complicated flow patterns controlled by the geometry, extent and connectivity of fractures and dissolution-enhanced conduits. In a bedrock river environment these features variably connect surface water to groundwater. Given the nature of discrete fracture and conduit networks, these flow systems can be anisotropic and heterogeneous over a wide range of scales. Portable and non-invasive geophysical methods are ideal for the initial characterization of shallow hydrogeologic systems in ecologically sensitive environments. Here, we evaluate the utility of the electromagnetic (EM) azimuthal resistivity method for characterizing shallow vertical and subvertical fracture set orientations in the presence of localized dissolution-enhanced features near a sedimentary bedrock river located in Southern Ontario, Canada. Multiple EM coil spacings ranging from 1 to 10m in vertical and horizontal dipole modes were applied at two locations within a 10×25m study plot; azimuthal rotations were conducted using symmetric and asymmetric geometries. The observed anisotropic response was evaluated using 100MHz ground-penetrating radar (GPR) measurements collected across the study plot area. A joint analysis of EM and GPR data revealed that fracture-based anisotropy can be identified in the presence of local heterogeneities (i.e., dissolution-enhanced features) at scales considerably smaller than those of previous studies. These non-invasive geophysical data sets can be used to optimize the design of multi-borehole groundwater monitoring stations along bedrock river channels, such that angled boreholes could be emplaced and orientated to intercept major fracture networks and local dissolution features. These data could also improve 3-D fracture network conceptualizations utilizing discrete information from angled and vertical boreholes.

Seasonal and spatial variations in rare earth elements to identify inter-aquifer linkages and recharge processes in an Australian catchment

With the aim of elucidating the seasonal behaviour of rare earth elements (REEs), surface and groundwaters were collected under dry and wet conditions in different hydrological units of the Teviot Brook catchment (Southeast Queensland, Australia). Sampled waters showed a large degree of variability in both REE abundance and normalised patterns. Overall REE abundance ranged over nearly three orders of magnitude, and was consistently lower in the sedimentary bedrock aquifer (18ppt<∑REE<477ppt) than in the other hydrological systems studied. Abundance was greater in springs draining rhyolitic rocks (∑REE=300 and 2054ppt) than in springs draining basalt ranges (∑REE=25 and 83ppt), yet was highly variable in the shallow alluvial groundwater (16ppt<∑REE<5294ppt) and, to a lesser extent, in streamwater (85ppt<∑REE<2198ppt). Generally, waters that interacted with different rock types had different REE patterns. In order to obtain an unbiased characterisation of REE patterns, the ratios between light and middle REEs (R(M/L)) and the ratios between middle and heavy REEs (R(H/M)) were calculated for each sample. The sedimentary bedrock aquifer waters had highly evolved patterns depleted in light REEs and enriched in middle and heavy REEs (0.17<R(M/L)<1.00 and −0.16<R(H/M)<0.93), whereas the springs draining intrusive and extrusive rocks had relatively flat patterns (0.20<R(M/L)<0.38 and −0.16<R(H/M)<0.09). Surface waters were generally enriched in middle REEs (median R(M/L)=0.35 and median R(H/M)=−0.04), and waters from the shallow alluvial aquifer had very diverse patterns with important spatial variations. Samples collected from the alluvium exhibited an increasing influence of the sedimentary bedrock from upgradient to downgradient; typically they showed flat patterns in the upstream section of the alluvium (median R(M/L)=0.21 and median R(H/M)=-0.06) gradually evolving towards patterns depleted in light REEs and enriched in middle and heavy REEs downgradient (median R(M/L)=0.48 and median R(H/M)=0.38). To document the seasonal variations in REE patterns, the difference in ratios between dry and wet sampling campaigns was determined for each repeated sampling location. Contributions from the sedimentary bedrock water to the alluvium during the wet season were identified at two locations (increase from R(H/M)=0.03 and 0.35 to R(H/M)=0.62 and 0.89). The effect of recharge through fractured igneous rocks was also observed in two boreholes intercepting the sedimentary bedrock, where the freshly recharged waters likely contributed to the deeper groundwater flow during the wet season (decrease from R(M/L)=0.81 and 0.56 to R(M/L)=0.46 and 0.17). Results from this study suggest that REEs may be usefully applied as indicators of recharge processes and inter-aquifer mixing. They also underline the importance of conducting seasonal sampling campaigns to capture possible short-term variations in REE patterns and abundance, which is essential to enable a better understanding of hydrological and hydrochemical processes in complex geological settings.

Exploration of Lead-Zinc (Pb-Zn) Mineralization Using Very Low Frequency Electromanetic (VLF-EM) in Ishiagu, Ebonyi State

Very low frequency Electromagnetic (VLF-EM) exploration over the Ishiagu area of Abakaliki Basin, Lower Benue Trough, Nigeria was carried out to determine the Pb-Zn mineralization in the sedimentary bedrock. The conductivity contrast between the conductive mineralized veins and the host rock as generated by induction mechanism was used to delineate the potential zones of Pb-Zn mineralization. Results of high in-phase and quadrature readings due to strong EM induction were detected in the survey area and on the average, the deeper sources response range from 4.7% to about 7.6%, while the shallower sources response range from 8.8% to about 17.1% and these probably indicate the presence of Pb-Zn deposits with thick overburden in the northern part. Current density maps show the Pb- Zn mineralized veins trends in NW-SE direction with their subordinates in N-S direction. The central part of the study area displays very sharp VLF tipper responses indicating shallow sources, while the northern part displays broad VLF tipper responses indicating deeper sources. These readings correlate to the depth values over the Pb-Zn mineralized veins which range from 10 to 13 m in the central part and 16 to 22 m in the northern part of the mapped area. The high VLF anomalies delineated from Ishiagu area, in the SW part of Abakaliki basin as a rule yielded a high conductivity contrast result well-suited with the Pb-Zn mineralization and geologic information of the area.

Groundwater Leakage and River Runoff in a Catchment Influenced by Tectonic Movement

In order to clarify how groundwater leakage and river runoff occur in a catchment under tectonic movement, the water balance was estimated in the forested (88.3% in area) Oikamanai River cat- chment (area, 62.6 km 2 ), Hokkaido, Japan. The catchment's geology is early Miocene to Pliocene sedimentary bedrock of partly high permeability, accompanied by currently active faults. Daily evapotranspiration, E, in water balance was calculated by applying the one-layer model to meteo- rological data in the rainfall season of 2011 and 2012, with the topographic influence on heat bal- ance of the catchment considered. The coupling with the short-term water balance method for riv- er runoff events allows us to estimate groundwater leaking to the other catchments through the faults and bedrock. As a result, the leakage corresponded to 50% - 80% of effective rainfall (=P − E: P, rainfall) in 2011, whereas it was lower or negative in 2012. The estimate of leakage then in- cluded variability of ca. 80%. In 2012, shallow groundwater storage seems to retain high baseflow during non-rainfall.

Stable Isotopes of Lithium as Indicators of Coal Seam Gas-bearing Aquifers

In this study lithium isotopes were used in combination with hydrochemistry to investigate interactions between coal-seam-gas bearing sedimentary bedrock aquifers and surrounding basalt and alluvial aquifers in a large catchment in eastern Australia. Understanding groundwater transport and aquifer connectivity is critical to managing coal seam gas (or coal bed methane) developments, because large volumes of water need to be extracted in order to release the sorbed gas; however, to date lithium isotopes have not been applied to coal seam gas groundwater management problems and no information on the δ7Li of coal or coal-seam groundwater is available. Li/Cl and Li/Na ratios in the coal-bearing and sedimentary bedrock aquifers are distinct (>0.0001) from alluvial and basalt aquifers (<0.0001). Preliminary δ7Li results for coal measure samples are typically between 7 and 11‰; many of these samples also contain methane, and can therefore be expected to be influenced by coal and the early stages methanogenesis. Interestingly the coal measure with lowest δ7Li value occurs in an area where the coal measures outcrop and direct recharge is likely, with nearby basalt groundwater having much higher δ7Li values (δ7Li>18‰). Preliminary lithium isotope results show that δ7Li may be effective in distinguishing groundwater flow paths in the coal-bearing aquifer from basalt aquifers, and from a transitional zone between the alluvium and underlying coal measures. Further lithium isotope analysis is being carried out to: a) compare the δ7Li between alluvial, basalt and coal-bearing aquifers to further investigate aquifer connectivity; b) to describe δ7Li for CSG production waters with low- and high-methane groundwater in the coal-bearing aquifer; c) to describe the δ7Li from coal and basalt leachate.

Quantification of effects of geometry of sediment bedrock interface on ground motion in 3D basin with circular free surface

Quantification of effects of geometry of sediment bedrock interface on ground motion in 3D basin with circular free surface

Hybrid Multilevel System for Monitoring Groundwater Flow and Agricultural Impacts in Fractured Sedimentary Bedrock

AbstractUnderstanding agricultural contamination in bedrock aquifers is challenging due to complexity of fracture networks and limitations in data acquisition imposed by instrumentation and drilling costs. Engineered assemblages known as multilevel monitoring systems (MLSs) maximize data from each borehole by providing numerous, depth‐discrete monitoring intervals for profiles of hydraulic head and hydrochemistry. This article describes a hybrid MLS that uses key components of the Waterloo MLS, with extra piezometers of sufficient diameter to accommodate removable transducers for continuous pressure monitoring, attached to the outside using custom clamps. Monitoring intervals are created with sand packs separated by bentonite seals, either via backfilling from surface or tremie placement. The hybrid MLS is best suited for use in rotary drilled boreholes 12 to 15 cm diameter, smaller holes have insufficient space for the MLS and added piezometer(s) while larger holes have excessive backfill material and purge requirements. Variations were installed to 60 m depth in sandstone in Prince Edward Island, and to 150 m in dolostone in southwestern Ontario. Transducers in the external piezometers provided temporal head data under ambient and stressed conditions in key intervals, and manual measurements in all ports provided detailed vertical hydraulic snapshots. Combined with hydrochemistry profiles from groundwater sampling, the hybrid MLS provided detailed composite datasets for interpreting flow and transport in these bedrock aquifers. The hybrid MLS offers promise as a versatile low‐cost option for groundwater studies in agricultural areas providing improved insights on groundwater flow systems and vertical distribution of nitrate and other contaminants, allowing more informed management decisions.

Geophysical conceptualization of a fractured sedimentary bedrock riverbed using ground-penetrating radar and induced electrical conductivity

Summary Bedrock rivers exhibit very different hydraulic and ecological regimes compared to alluvial rivers. Groundwater–surface water interaction in rivers that flow directly on sedimentary bedrock surfaces with exposed fracture and conduit networks is generally based on alluvial river conceptual models. However, the dual and triple porosity systems of sedimentary rock (e.g., matrix, fractures and vugs) leads to highly variable hydraulic pathways and groundwater velocities that, in turn, result in processes that are very different than those derived from alluvial concepts. Relative to alluvial rivers there has been very little direct examination of dynamic interactions, such as fluxes, groundwater velocities, and mechanisms affecting water quality across the channel interface; this is largely because bedrock rivers are difficult to instrument and monitor given their heterogeneous and anisotropic flow systems. In this study, we evaluate the capacity of non-invasive, low impact ground-penetrating radar and frequency-domain electromagnetic induction methods capable of detecting discrete fracture and conduit features, mapping complex distributions of lithostratigraphic interfaces, and providing information about formation properties along a partially exposed section of the Eramosa River located in Ontario, Canada. Geophysical data are supported by direct hydrogeologic and geologic information obtained from a nearby borehole piezometer cluster and a continuous 15 m rock core. Valuable insights were gained into the lateral extent and geometry of structurally-controlled whydrostratigraphic units (e.g., dissolution-enhanced fractures, epikarst and karst features), which are expected to strongly influence groundwater and surface water interaction along the channel. An integrated application of GPR and EMI methods was necessary to fully understand the complex geometry of geologic boundaries and karst features within the shallow bedrock aquifer. The identification of epikarst below fluvial sediment and its relationship to the formation of dissolution features, exposed fracture conduit networks and sinkholes represents the basis for understanding the dynamic hydraulic responses between the surface water and the shallow bedrock aquifer.

Electrical Resistance Heating of Volatile Organic Compounds in Sedimentary Rock

Electrical resistance heating (ERH) is a common method of remediation for volatile organic compounds in unconsolidated soils, both above and below the water table. In the past eight years, ERH has been used to successfully treat 10 or more contaminated sedimentary bedrock sites. Sedimentary bedrock treatment has recently expanded to greater depths and into karst limestone environments. This article describes the implementation issues for rock remediation and provides case studies of three sites remediated by ERH in Pennsylvania and Alabama. With proper design, the remediation of sedimentary bedrock can be completed as effectively as the remediation of overburden materials. © 2014 Wiley Periodicals, Inc.

Assessing dissolved methane patterns in central New York groundwater

Abstract Study region Groundwater in Chenango County (central New York State, USA), which is underlain by Devonian sedimentary bedrock. This region has conventional natural gas wells and is under consideration for future shale gas development using high-volume hydraulic fracturing. Study focus The study examines current patterns of dissolved methane in groundwater, based on 113 samples from homeowner wells in the spring of 2012. Samples were analyzed for methane and other water quality parameters, and each well characterized by its landscape position and geology. Statistical testing and regression modeling was used to identify the primary environmental drivers of observed methane patterns. New hydrological insights for this region There was no significant difference between methane concentrations in valleys versus upslope locations, in water wells less than or greater than 1 km from a conventional gas well, and across different geohydrologic units. Methane concentrations were significantly higher in groundwater dominated by sodium chloride or sodium bicarbonate compared with groundwater dominated by calcium bicarbonate, indicating bedrock interactions and lengthy residence times as controls. A multivariate regression model of dissolved methane using only three variables (sodium, hardness, and barium) explained 77% of methane variability, further emphasizing the dominance of geochemistry and hydrogeology as controls on baseline methane patterns.

Rapid channelization and incision into soft bedrock induced by human activity — Implications from the Bachang River in Taiwan

Many Taiwanese river channels have been affected by human activity, such as engineering works and gravel mining. Environmental conditions such as high seasonal precipitation and young sedimentary bedrock exaggerate the narrowing and incising of river channels. The Bachang River is located in southwestern Taiwan, and the 11-km-long channel at its midstream reach has been incised to a maximum depth of approximately 30m within several decades. Therefore, the morphologic evolution of the Bachang River in southwestern Taiwan provides an illustration of channel changes in response to human activity. The historical data used for this analysis of morphology change include one set of topographic maps and six sets of aerial photographs from the past 100years. The channel changes are analyzed qualitatively and quantitatively and include the channel planform, longitudinal profiles, channel incision, channel width, and channel cross sections. The results indicate that remarkable channel changes have occurred since the 1980s as a result of human influence. The 11-km-long study reach transformed from an alluvial-type channel to a gorge-type channel within several decades. The maximum accumulated depth of the channel incision is approximately 30m, with a meter-scale annual average incision rate; the maximum width has been decreased to approximately one-sixth of the original width (448m). The process of channel evolution is divided into four stages, and we have concluded that the causes of channel change include six factors from two categories: natural factors that include geological conditions, hydrological conditions, and the process of bedrock erosion; and human factors that include gravel mining, lateral structures, and levees. The initiation of channel evolution is triggered by human factors. Finally, we discuss the potential future channel evolution and lessons learned from the case study.

Hydrogeological Importance of Bedrock Sediments to the Community and Growth of Sugar Cane in Fadama Rake Area of Madagali, Northeast Nigeria

Thirty Vertical Electrical Sounding (VES) points were sited on the alluvial plain of Madagali fadama area. This was aimed at establishing the lithological character of the drainage plain and its contributions to the growth of sugar cane. 1X1D shareware package was used to interpret the curves. Over the floodplain, potential recharge water into the subsurface units probably takes place through migration routes of coarse-grained colluvial deposits which act as effective soak away for surface runoff. The nature of the basin and/or the transporting routes conditioned the thickness of the surface layer. Effects of near surface bedrock or buried granite boulders disengaged from nearby hills are demonstrated by sandwiching of the bedrock in place in the second resistivity layer. Closely spaced iso-ohmic contour values suggest presence of different types of sediments within the medium of deposition. The high thickness associated with this horizon suggests that the basin was large enough to contain the sediments. Clay materials in the third resistivity layer occur as lens bodies within sandy material. Clay free sand and gravel constitute a great proportion of the stratigraphy. Parts of the bedrock

Evaluating shallow flow-system response to climate change through analysis of spring deposits in southwestern Wisconsin, USA

Tufa-depositing springs in the southern Driftless Area of Wisconsin, USA, are used to inform the response of shallow and local groundwater flow systems to changes in climate over the last 3,000 years. The springs emanate from a shallow, unconfined, sedimentary bedrock aquifer and at stratigraphic positions similar to a shallow, perched aquifer that was identified in the eastern Driftless Area. The perched aquifer was shown to be stable under current climate conditions and over decadal time scales. This study provides further evidence of the significance of the stratigraphic interval in controlling shallow groundwater flow patterns in the region and in the stability of shallow and local groundwater flow systems over thousands of years. The tufa carbonates in three cores collected from the mounds adjacent to the springs show variations in stable isotope (δ13C, δ18O) and elemental (Mg/Ca) values that agree with well-established paleoclimate records for the region, suggesting that the springs were active and depositing tufa in the past, during climate conditions that were similar to the present and during conditions that were drier than the present.

Eolian contribution to geochemical and mineralogical characteristics of some soil types in Medvednica Mountain, Croatia

Six pedological profiles were analyzed above five typical lithological units in Medvednica Mountain to determine the effect of eolian additions to the soil composition, as well as the possible influence of relief, vegetation cover, and anthropogenic input on the dynamics of pedogenesis. Bedrock composition was defined using petrographic (thin sections) and chemical analyses (major and trace element contents), whereas pedological, sedimentological and geochemical characteristics of six cross-sections were determined by chemical (major and trace element contents), mineralogical (modal analysis), and grain size analysis. Soils developed on sedimentary bedrock (Mollic Rendzic Leptosols (Calcaric) and Albic Luvisol (Sceletic)) mostly originate from weathering of siliciclastic detritus that was exposed to oxidization before diagenesis. Furthermore, soils on metasiltstone, marble and Lithothamnium limestone exhibit similar modal and geochemical composition and element distribution across the soil profile. In contrast, soils developed on igneous bedrock originate mostly from weathering of minerals of the first weathering cycle and thus abound with chemically less resistant minerals and less mobile elements. However, all profiles contain higher concentrations of lead with respect to bedrock indicating airborne contamination.

New data on the palaeo–incisions network of the south–eastern Baltic Sea

The present study area is located within the south–eastern segment of the Baltic Sea framed by 55o30’–56o30’ N and 19o00’–21o15’E. The area is re-visited with the aim to describe in more detail the geologic prerequisite and development of the palaeo–incisions as well as the timing of their subsequent infillings. The channels form distinctive features in the sedimentary bedrock along the outer limits of pre–Weichselian ice sheets, on average reaching depths into the bedrock of 50 m in the nearshore zone of Lithuania to 100 m along the slope to the Gotland depression in the west. The development of palaeo–incisions systems is governed by the easily eroded late Palaeozoic to Mesozoic bedrock of the present area. Only rare ocurrences of channels have been reported from the middle and lower parts of the Palaeozoic further west in the Baltic Sea. The present investigation supports a mechanism that the channels formed below the ice near the ice sheet margin by melt water erosion under high pressure. The channels start at random where a fracture in the ice develops forming outlet of water contained below the central part of the ice sheet. The channels often merge together in the direction of the ice margin, possibly gradually adapting to previous fracture systems in the bedrock. The investigated incisions were infilled prior to the advance of the Weichselian ice sheet and some have been reopened and repeatedly infilled.

Joint inversion of spatial autocorrelation curves with HVSR for site characterization in Newcastle, Australia

In order to investigate site characterization of Newcastle affected by the 1989 Newcastle Earthquake (ML = 5.6), we conducted microtremor array measurements. The spatial autocorrelation (SPAC) method was applied to estimate the S-wave velocity structure from observed microtremor data. Although the inversion of spatial autocorrelation curves is effective for estimating shallow S-wave velocity structures within the sedimentary layer, it is usually difficult to estimate the boundary between the sedimentary layer and bedrock due to a lack of amplitude of vertical components of microtremors at low frequencies. On the other hands, it is well known that horizontal to vertical particle motion spectral ratios (HVSR) have information which assists in resolving the bedrock depth and velocity. Thus, we have applied joint inversion of the spatial autocorrelation curves with HVSR to observed microtremors. Since observed HVSR curves are subject to fluctuations due to unknown Love and body wave contributions or other noise effects, there is difficulty in fitting absolute values of observed HVSR. Therefore, we evaluate observed HVSR and theoretical H/V spectra by zero-lag cross-correlation to fit the shapes of HVSR. The observed HVSR curve has accurate information for the deeper velocity structure and therefore, the estimated velocity model by joint inversion differs in velocity estimations at depth. It is concluded that the joint inversion of the spatial autocorrelation curves with HVSR is useful in practice for obtaining improved estimates of S-wave velocity models down to bedrock.

Natural Buffering of Groundwater Contaminants Related to Development in Fractured Rock

AbstractIn rural areas of New England groundwater from fractured crystalline and sedimentary bedrock is a critical water resource. Increasingly, studies have shown that development occurring in rural areas is resulting in the impairment of water quality in fractured rock aquifers. The objective of this study was to evaluate the spatial and temporal variations in groundwater quality associated with development and evaluate the extent to which common groundwater contaminants associated with rural development may be naturally buffered. The study entailed a compilation and synthesis of over 2500 reports on domestic water quality that spanned a 30 year period. Focus was placed on the spatial distribution and temporal variations in sodium, chloride, iron, manganese, nitrate, and nitrite. Results indicate that despite significant levels of development, the amount of contamination to the bedrock has been minimal. Of the constituents examined, only the chloride concentration exhibits a systematic increase over time, but the level of chloride remained relatively low. The flux of chloride to the bedrock from deicing appears minimal despite the significant amounts of road salt used in the study area. Sodium concentrations in the bedrock remained relatively constant and appear to be buffered by ion exchange with calcium as suggested by the increase in hardness with time. Iron and manganese were present at relatively low levels but did not show any systematic trends over time. Nitrate and nitrite concentrations were very low and found to be inversely correlated with manganese and iron concentrations. This suggests that the presence of iron and manganese contribute to denitrification. This study indicated that both geochemical and biogeochemical processes are active buffering mechanisms that help shield the bedrock from contaminants associated with development.