Compressed high-intensity radiated pulse (CHIRP) acoustic reflection data acquired in lakes and other shallow water bodies can provide detailed imaging that is valuable for environmental and engineering geology applications well below the sub-bottom floors. However, it is often unclear to non-specialists which acquisition settings should be considered to achieve optimal imaging quality when beginning a new survey, particularly in an unfamiliar environment. We review fundamental concepts for optimizing signal design using coincident CHIRP lines from two extant glacial lakes in the eastern Sierra Nevada of California: Convict Lake and June Lake. As part of the optimization process, seismic data were recorded with different source acquisition parameters, including sweep bandwidth and length, in pre-production testing to determine the most effective acquisition parameters for satisfying survey depth and resolution objectives. We offer recommendations for best practices based on these results.
Persistent seepage from an earthfill dam and associated structures has been observed for decades at Lake Capote, a small, spring-fed, off-stream, manmade lake in southwestern Colorado. Extensive engineering measures in 2006 did not fully remediate seepage. Therefore, we employed electrical resistivity tomography (ERT), induced polarization (IP), and self-potential (SP) methods to determine the location of seepage and its relationships with local geologic conditions. ERT revealed an approximately north-south striking horizontal, high-resistivity feature extending underneath both the northern part of the dam and the adjacent south dike. IP results indicated that this interpretive feature exhibits low chargeability (i.e., a low ability to store electrical charges), and SP results identified localized high positive magnitudes in it. In the context of borehole data and published literature, ERT and IP results suggest that this feature is a rare sandstone bed in the local shale-dominated bedrock (Lewis Shale) and also that it is the location of persistent seepage. Seepage appears to be occurring under the dam along faults in the interpretive sandstone (similar faults are observable in a nearby outcrop) and also within highly fractured shale under the south dike. In contrast, the middle part of the dam shows evidence, in both ERT and IP results, for seepage through its embankment rather than in its foundation. Our results demonstrate the value added to forensic investigations of dams by the integration of different geophysical methods, and they also portend the crucial importance and cost-saving potential of preconstruction geophysical site characterization of damsites.
Degradation of urban surface water quality has increased the importance of understanding the spatial and temporal distribution of pollutants within aquatic systems. Poor water quality damages aquatic habitats and poses a human health risk. The purpose of this study was to investigate environmental and anthropogenic factors that impact water quality in the Rock Creek Watershed in Maryland and the District of Columbia by monitoring microbiological (E. coli and total coliforms), chemical, and physical water quality parameters. Sampling weekly and following >13 mm rain events during a four-month period obtained 295 total water samples. The geometric mean E. coli concentration for each month exceeded the permissible standard. Spatial and temporal data analysis using Geographic Information Systems software and statistical analysis determined that water quality is not uniform along Rock Creek. Areas with more than 50 percent agricultural and low-density land use had higher mean nitrate concentrations but lower electrical conductivity and phosphate concentrations compared with urbanized areas with a high density of older, potentially leaking sewers. Impervious surface coverage was positively correlated with mean electrical conductivity and negatively correlated with mean nitrate concentration. Precipitation and creek discharge rate were positively correlated with mean enteric bacteria concentration. Temporal (distribution of rainfall) as well as spatial (pet waste from dog walking) influences may impact the dynamics of microbial contamination in the watershed. These data may assist decision makers in understanding the relationship between water quality of Rock Creek, the factors studied, and the potential health hazards resulting from precipitation events.
This study aimed to predict the soil erodibility factor (K factor) using visible and near-infrared (VNIR) spectroscopy coupled with a partial least-squares regression (PLSR) model and to integrate the predicted K factor into the RUSLE model for improved soil erosion assessment. The K factor was calculated based on the Universal Soil Loss Equation (USLE) model and also predicted using VNIR spectroscopy coupled with the PLSR model. The combination of these factors generated a GIS map illustrating soil loss. Our results revealed a significant negative correlation between soil permeability and the USLE K factor (r =-0.75, p < 0.05). Conversely, percent silt showed the strongest positive correlation with the K factor (r = 0.55, p < 0.05). Results also showed that the K factor had significant correlations with several spectral bands, including 450 nm(r=0.36, p<0.05), 641 nm(r=-0.21, p<0.05), 990 nm (r = 0.26, p < 0.05), and 1,910 nm (r = 0.22, p < 0.05). Notably, the mean VNIR-PLSR K factor (0.28 t hr MJ(-1) mm(-1)) was twice that of the USLE K factor (0.13 t hr-1 MJ(-1) mm(-1)). The average soil loss predicted by the RUSLE model using the VNIR-PLSR K factor was 39.2 t hr(-1) yr(-1), i.e., more than double the average soil loss estimated using the USLE K factor (18.6 t hr(-1) yr(-1)). Soil loss ranged from negligible in flat areas to over 60 t ha-1 yr-1 in mountainous regions. Approximately 20 percent of the study area experienced very low to low soil loss, while another 20 percent faced very severe soil erosion.
The Southern High Plains are bounded on the west through the southern half of eastern New Mexico by the roughly linear Mescalero Ridge, an erosional, westward-facing near vertical escarpment, up to 150 ft (45.7 m) in height, capped by the 10 to 40 ft (3 to 12.2 m) thick caprock caliche. As it approaches the Texas state line, it loses linearity and topographic definition, developing numerous re-entrants and significant erosional cuts and gullying. Over this section, it is underlain or immediately adjacent to a corresponding ridge on the buried Triassic erosional surface termed the red bed ridge, a drainage divide throughout the Cenozoic. The red bed ridge and the Mescalero Ridge continue eastward into Andrews County with the Mescalero Ridge as the topographic drainage divide between the Colorado and Pecos Rivers, turning south and blending into the Cretaceous rocks of the Edwards Plateau. The buried red bed ridge is on the Triassic erosional surface of the upper part of the Dockum Group mud-stones, the Cooper Canyon Formation. The red bed ridge is at least partially of structural origin in the vicinity of the Waste Control Specialists facility just east of the New Mexico/Texas state line. A similar anticlinal structure occurs in underlying Permian units over a thickness of several thousand feet. Several commercial facilities have been developed in the state line area with the near-surface proximity of the low permeability Triassic mudstones providing isolation from potable groundwater and reliable waste containment characteristics for hazardous and radioactive wastes.
