Investigating local variation in groundwater recharge along a topographic gradient, Walnut Creek, Iowa, USA

Groundwater recharge is an important component to hydrologic studies but is known to vary considerably across the landscape. The purpose of this study was to examine 4 years of water-level behavior in a transect of four water-table wells installed at Walnut Creek, Iowa, USA to evaluate how groundwater recharge varied along a topographic gradient. The amount of daily water-table rise (WTR) in the wells was summed at monthly and annual scales and estimates of specific yield (Sy) were used to convert the WTR to recharge. At the floodplain site, Sy was estimated from the ratio of WTR to total rainfall and in the uplands was based on the ratio of baseflow to WTR. In the floodplain, where the water table is shallow, recharge occurred throughout the year whenever precipitation occurred. In upland areas where the water table was deeper, WTR occurred in a stepped fashion and varied by season. Results indicated that the greatest amount of water-table rise over the 4-year period was observed in the floodplain (379 mm), followed by the upland (211 mm) and sideslopes (122 mm). Incorporating spatial variability in recharge in a watershed will improve groundwater resource evaluation and flow and transport modeling.     

Late Holocene vegetational history,human impact and pollen representativity variations in northern Cumbria,England

The comparison of pollen diagrams and their inferred vegetational histories are an important component of palaeoecological research. Radiocarbon-dated pollen profiles from three cores taken from two adjacent mires located in northern Cumbria, Bolton Fell Moss and Walton Moss, have been used to reconstruct the Late Holocene vegetation history between the Bronze Age and the present day. The profiles have been interpreted in the light of available archaeological and historical records and, although the pollen records are broadly similar, there are some notable differences between them, particularly during Iron Age and medieval times. Dissimilarities between the diagrams are explored numerically, and the statistical and palynological results are discussed in relation to pollen representativity. The results suggest that it may be advantageous to construct more than one pollen diagram from a mire, or even adjacent mires, as extra-local pollen may be a more important part of the pollen rain than previously envisaged. © 1998 John Wiley & Sons, Ltd.     

Petrology and mineral chemistry of sedimentary rocks from the Western Solomon Sea

Multichannel seismic reflection profiles recorded in the northern Red Sea show structures that we interpret to be a result of the intrusion of uppermost Miocene salt. We believe that the evaporites are underlaid by attenuated continental crust and the flow of salt is due to renewed faulting of basement in the Pliocene when sea floor spreading began between latitudes 21°N and 15°30°N.     

The rate of denudation of some British lowland landscapes

Developments in dating techniques applicable to the late Tertiary and Quaternary are giving us the ability to date past land surfaces. Where reasonable assumptions about the nature of such past surfaces and their partial preservation may be made, they can be reconstructed. This permits the contouring and measurement of the subsequent dissection, allowing not only calculation of the average rate of erosion over the elapsed time, but also information on the pattern of incision. Two examples where this has been attempted are present; both are dissected till surfaces in eastern England, one of Anglian and the other of Devensian age. The approach quantifies the disparity between the incision of valleys and the general denudational lowering of the surface which characterizes many landscapes. The technique is not only of academic interest, but potentially forms a useful line of approach to the assessment of the safety of the burying toxic wastes. © 1997 by John Wiley & Sons, Ltd.     

Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis

A vertical profile of maximum horizontal principal stress, SHmax, orientation to 5 km depth was obtained beneath the Swiss city of Basel from observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 m exploration well (OT2) imaged from 2550 m to 2753 m across the granitic basement-sediment interface at 2649 m; and a 5 km deep borehole (BS1) imaged entirely within the granite from 2569 m to 4992 m. Stress-related wellbore failure in the form of breakouts or drilling-induced tension fractures (DITFs) occurs throughout the depth range of the logs with breakouts predominant. Within the granite, DITFs are intermittently present, and breakouts more or less continuously present over all but the uppermost 100 m where they are sparse. The mean SHmax orientations from DITFs is 151 ± 13° whereas breakouts yield 143 ± 14°, the combined value weighted for frequency of occurrence being N144°E ± 14°. No marked depth dependence in mean SHmax orientation averaged over several hundred meters depth intervals is evident. This mean SHmax orientation for the granite is consistent with the results of the inversion of populations of focal mechanism solutions of earthquakes occurring between depths of 10–15 km within regions immediately to the north and south of Basel, and with the T-axis of events occurring within the reservoir (Deichmann and Ernst, this volume). DITFs and breakouts identified in OT2 above and below the sediment-basement interface suggest that a change in SHmax orientation to N115°E ± 12° within the Rotliegendes sandstone occurs near its interface with the basement. The origin of the 20–30° change is uncertain, as is its lateral extent. The logs do not extend higher than 80 m above the interface, and so the data do not define whether a further change in stress orientation occurs at the evaporites. Near-surface measurements taken within 50 km of Basel suggest a mean orientation of N–S, albeit with large variability, as do the orientation of hydrofractures at depths up to 850 m within and above the evaporite layers and an active salt diapir, also within 50 km of Basel. Thus, the available evidence supports the notion that the orientation of SHmax above the evaporites is on average more N–S oriented and thus differs from the NW–SE inferred for the basement from the BS1/OS2 wellbore failure data and the earthquake data. Changes in stress orientation with depth can have significant practical consequences for the development of an EGS reservoir, and serve to emphasise the importance of obtaining estimates from within the target rock mass.     

Cytochrome P4501A biomarker indication of the timeline of chronic exposure of Barrow's goldeneyes to residual Exxon Valdez oil

We examined hepatic EROD activity, as an indicator of CYP1A induction, in Barrow’s goldeneyes captured in areas oiled during the 1989 Exxon Valdez spill and those from nearby unoiled areas. We found that average EROD activity differed between areas during 2005, although the magnitude of the difference was reduced relative to a previous study from 1996/1997, and we found that areas did not differ by 2009. Similarly, we found that the proportion of individuals captured from oiled areas with elevated EROD activity (?2 times unoiled average) declined from 41% in winter 1996/1997 to 10% in 2005 and 15% in 2009. This work adds to a body of literature describing the timelines over which vertebrates were exposed to residual Exxon Valdez oil and indicates that, for Barrow’s goldeneyes in Prince William Sound, exposure persisted for many years with evidence of substantially reduced exposure by 2 decades after the spill.