Cluster analysis of a regional-scale soil geochemical dataset in northern California

A regional-scale soil geochemical study was conducted within a 22,000 km² area in northern California including the Sierra Nevada, Sacramento Valley, and northern Coast Range. Over 1300 soil samples were chemically analyzed for 42 elements. The distribution of distinct groups of elements demonstrates the interplay of geologic, hydrologic, geomorphologic and anthropogenic factors; however, it is difficult to fully appreciate the complexity of geochemical transport and weathering processes on a landscape-scale in an area of very complex geology with such a large dataset containing more than 40 variables. To examine the data from a perspective of multi-element groupings, cluster analyses were applied to the dataset. The analysis identified several groups of elements whose spatial patterns could be related to specific geologic sources.     

Factors affecting the distribution of benthic macrofauna in estuaries contaminated by urban runoff

Contaminants derived from urban runoff have been shown to accumulate in estuarine sediments, reaching concentrations potentially capable of causing biological effects. Demonstration of effects, however, is difficult due to strong natural environmental gradients and the effects of past or present point-sources of contamination. We used multivariate methods to test two hypotheses relating to the effects of urban-derived contaminants on estuarine benthic communities. First, that patterns of distribution and abundance of benthic invertebrates in two urbanised estuaries would be different from those in two non-urbanised estuaries. Second, that the distributions of benthic invertebrates within and among the four estuaries would be related to those of urban-derived contaminants. Concentrations of contaminants were larger in estuaries with urbanised catchments and concentrations of Cu, Pb, Zn and DDT in some samples exceeded those at which biological effects may be expected to appear. Tests of differences in composition of benthic communities among estuaries showed that the two urban estuaries were not significantly different, but that they differed from both rural estuaries, which also differed from each other. Distributions of benthic invertebrates were significantly related to those of environmental variables, and were ordinated along axes that correlated with both natural environmental variables (nature of the sediment, position in estuary) and contaminants. Differences in faunas between the urban and non-urban estuaries were not, however, clear-cut and nor were relationships between faunal assemblages and environmental variables (including contaminants) consistent between two times of sampling.     

Evolution of undercut slopes on abandoned incised meanders in the Eastern Highland Rim of Tennessee, USA

Field surveys, location-for-time reasoning, and computer modeling were used to study the evolution of slopes on valley walls of abandoned bedrock meanders on the Eastern Highland Rim, Tennessee. Hillslopes on the undercut slopes of cutoff incised meanders were ordered as to relative age by the height of their meander floors above the modern stream level. The assumption is that the undercut slope is actively eroded by the stream until abandonment of the meander, at which time the slope begins to evolve to a different form. More-advanced stages of evolution occur on walls of higher meanders that were abandoned earlier. The most rapid change in this initial form is the elimination of a free face, which occurs soon after the meander is abandoned. In addition, the hillslopes associated with even the lowest (youngest) cutoff meanders show somewhat gentler overall gradients than the actively undercut slopes. Hillslopes associated with meanders 3 to about 20 m above modern stream level maintain straight segments with angles showing only a slight decrease from the 36–38° associated with the lowest cutoffs; overall angles decrease, however, as the straight segment becomes shorter. The oldest slopes, those on cutoffs 30 m or greater above modern stream level, have developed into convex–concave slopes with maximum slopes of 15°.A hillslope evolution model based on previously published algorithms was used to simulate the transition of actively undercut hillslopes into hillslopes on abandoned meanders. Hillslope modeling is particularly useful in this setting. If the valley incision rate is known, an age can be estimated for the cutoff and hence for the hillslope. Alternatively, if hillslope process rates are known, a model age obtained for the hillslope can be used to estimate an incision rate. Even where both incision rates and hillslope process rates are poorly constrained, as in the present setting, modeling allows assumptions about specific rates to be evaluated by determining their implications for other rates. For example, for three cutoff meanders along one stream, best-fit criteria were used to select process rates for the model. Model ages of hillslopes were then obtained and compared with those calculated from a valley-incision rate measured elsewhere in the same physiographic province. For two of the hillslopes, model ages were found to be much younger than those calculated from the incision rate. In order to make the two ages agree, unreasonably low process rates had to be used in the model, thus implying that the incision rate probably underestimates the actual incision rate in this valley.Experimentation with heights of initial profiles, again using best-fit criteria, suggests that since abandonment of the highest cutoff, the plateau has been downwasting at a rate about one-fourth that of the valley incision rate, a finding in agreement with published rates of chemical denudation in the area.     

Evaluation of bioremediation strategies of a controlled oil release in a wetland

A controlled petroleum release was conducted to evaluate bioremediation in a wetland near Houston, Texas. The 140-day study was conducted using a randomized, complete block design to test three treatments with six replicates per treatment. The three treatment strategies were inorganic nutrients, inorganic nutrients with an alternative electron acceptor, and a no-action oiled control. Samples were analyzed for petroleum chemistry and inorganic nutrients. These results are discussed in the context of our related research involving toxicology and microbiology at the site during the experiment. To evaluate biodegradation, the targeted compounds were normalized to the conservative compound C3017alpha, 21beta-[H]hopane, thus reducing the effects of spatial heterogeneity and physical transport. The two biostimulation treatments demonstrated statistically-higher rates of biodegradation than the oiled no-action control. For the majority of the experiment, target nutrient levels were maintained. Further research may be warranted to optimize these bioremediation strategies as well as evaluating additional treatment strategies for wetlands and other shoreline systems.     

The Role of Mid-Latitude Pacific Cyclones in the Winter Precipitation of California*

The characteristics of the mid-latitude Pacific cyclones that produce precipitation over California are investigated statistically to determine their role in governing precipitation receipt. From all cyclones occurring over the Pacific Ocean from December through March and between 1965–1990 inclusive, we identify a subset of cyclones likely to produce precipitation over California. The characteristics analyzed are track, frequency, duration, speed, central pressure, and proximity. These are related to monthly precipitation for the entire period and are also used to explain the difference in precipitation received during the earlier (wet) and latter (dry) halves of the 1980s. Results indicate that the winter cyclones responsible for precipitation over California originate in the southeast quadrant of the region influenced by the Aleutian Low and decay south of the Gulf of Alaska. The extent of cyclonic activity over the east Pacific Ocean near California diminished during the dry period. There were fewer cyclones, they were significantly weaker, and they traveled along a more meridional track during the dry period.     

A reevaluation of the oceanic uranium budget for the Holocene

We present a new assessment of the pre-anthropogenic U budget for the Holocene ocean. We find that the gross input of U to the ocean lies in the range 53±17 Mmol/year, where the dominant source is river runoff (42.0±14.5 Mmol/year) and the direct discharge of groundwater could represent a significant additional input (9.3±8.7 Mmol/year). The soluble U flux associated with the aeolian input of crustal dust is minor (1.8±1.1 Mmol/year), falling well within the errors associated with the riverine flux. Removal of U to the organic rich sediments of salt marshes and mangrove swamps during river–sea mixing may significantly modify the riverine flux, such that the net U input is reduced to 42±18 Mmol/year. Evaluation of the U isotope budget demonstrates that the limits we have established on the U input flux are reasonable and suggests that direct groundwater discharge may play a significant role in maintaining the oceanic excess of ²³⁴U. The total sink of U from the ocean lies in the range 48±14 Mmol/year. We find that three major processes control the magnitude of this flux: (1) removal to oxygen-depleted sediments (26.9±12.2 Mmol/year); (2) incorporation into biogenic carbonate (13.3±5.6 Mmol/year); and (3) crustal sequestration during hydrothermal alteration and seafloor weathering (5.7±3.3 Mmol/year). The removal of U to opaline silica (0.6±0.3 Mmol/year) and hydrogenous phases (1.4±0.8 Mmol/year) is minimal, falling well within the errors associated with the other sinks. That the input and output fluxes balance within the calculated errors implies that U may be in steady state in the Holocene ocean. In this case, the input and output fluxes lie in the range 34–60 Mmol/year, giving an oceanic U residence time of 3.2–5.6×10⁵ years. However, given the large uncertainties, a significant imbalance between the Holocene input and output fluxes cannot be ruled out. The constancy of the ancient seawater U concentration implies that the U budget is in steady state over the time period of a glacial–interglacial climate cycle (10⁵ year). A Holocene flux imbalance must, therefore, be offset by an opposing flux imbalance during glacial periods or at the interglacial–glacial transition. We suggest that the storage of U in the coastal zone and shallow water carbonates during interglacial periods and the release of that U at or following the interglacial–glacial transition could be sufficient to affect the short-term stability of the U budget. Providing tighter constraints on U fluxes in the Holocene ocean is a prerequisite to understanding the U budget on the time scale of a glacial–interglacial climate cycle and using this element as a valuable palaeoceanographic proxy.     

Field measurement and associated controlling factors for ground water seepage in a piedmont impoundment

Knowledge of groundwater seepage to and from lakes can be an important component of scientific investigations involving water and geochemical budgets. Measurements of groundwater seepage at Lake Anna, a man-made lake in central Virginia, show that inflow to the lake occurs even under dry summer conditions. Seepage rates were found to be correlated with the elevation of the near-shore water table, which responded rapidly to rainfall events in the fractured rock terrane in which Lake Anna is located. Seepage rates did not decline uniformly with distance offshore. This result contrasts with those for lakes underlain by relatively homogeneous porous media where measurements generally confirm the prediction that seepage rates drop off exponentially with distance from shore. The along shore variability of seepage rates in Lake Anna was related to a topographic index that is used to describe drainage from hillslopes. This suggests that seepage in impoundments such as Lake Anna may be strongly controlled by drainage pathways that pre-date the lake.