Water Table Drawdown Research Articles

On the geochemistry of gases and noble gas isotopes (including 222Rn) in deep crustal fluids: the 4000 m KTB‐pilot hole fluid production test 2002–03

AbstractThe concentrations of H2, O2, CO2, and concentrations and isotopic composition of the noble gases (including 222Rn), N2, CH4, and higher hydrocarbons dissolved in 4000 m deep‐seated fluids from a 12‐month fluid production test in the KTB pilot hole were analyzed. This determination of the gas geochemistry during the test in combination with the knowledge of the hydraulic data provides relevant information about the fluid hydraulics of the deep system. All gas concentrations and isotopic signatures, except for 222Rn, showed constancy during the course of the test. This, in combination with large fluid flow rates at a moderate water table drawdown, imply an almost infinite fluid reservoir in 4000 m depth. From the change in 222Rn‐activity as a function of pump rate, the contribution of smaller and wider pores to the overall fluid flow in an aquifer can be deduced. This 222Rn‐activity monitoring proved therefore to be a valuable instrument for the qualitative observation of the scavenging of pore and fracture surfaces, a hydraulic feature invisible to standard hydraulic testing tools. The observance of this scavenging effect is due to (i) the continuous on‐line geochemical monitoring, (ii) the durability of the test, (iii) a change in pump rate during the course of the test, and (iv) due to the short half‐life of 222Rn. The fluids have a 5.9% mantle He component, and a δ21Ne excess of 14%, and a noble gas model age of about (5.5–6.2) ± 2.0 Myr. The mean N2/Ar‐ratio of 516 and δ15N‐data of about +1.5‰ indicates sedimentary or metamorphic origin of N2. The hydrocarbons, amounting to 33 vol.% in the gas phase, are derived from thermal decomposition of marine organic matter of low maturity. But a key question, the identification of the potential source region of the fluids and the migration pathway, is still unidentified.

METHODS TO DETERMINE LATERAL EFFECT OF A DRAINAGE DITCH ON WETLAND HYDROLOGY

A method was developed to estimate the lateral effect of a single drainage ditch on wetland hydrology. The methodcan be used to calculate the distance of influence of a single ditch constructed through a wetland, where the distance ofinfluence is defined as the width of a strip adjacent to the ditch that is drained such that it will no longer satisfy the wetlandhydrologic criterion. Simulation analyses were conducted with DRAINMOD to define the minimum, or threshold, drainageintensity that would result in failure of a site to satisfy the wetland hydrologic criterion. Analyses were conducted for fivehydric soils spanning a wide range of profile hydraulic transmissivities. DRAINMOD was used to predict water tablefluctuations between parallel ditches for a 50-year period of climatological record. For each soil, simulations were conductedfor a range of ditch spacings and depths to determine the combinations that would result in the land midway between theditches just barely satisfying the wetland hydrologic criterion. Analyses were conducted for climatological conditions forthree locations in eastern North Carolina. Results for Wilmington, North Carolina, showed that the threshold drainageintensities would result in water table drawdown from an initially ponded surface to a depth of 25 cm in approximately 6 days.That is, ditch depths and spacings sufficient to lower the water table from the surface to a depth of 25 cm in a threshold timeof about 6 days would result in hydrologic conditions that would just barely satisfy the wetland hydrologic criterion for thatlocation. The threshold time is denoted T25 and is used as a surrogate for quantifying the water table drawdown rate of sitesthat barely satisfy the wetland hydrologic criterion. T25 was found to depend somewhat on drain depth, but it was essentiallyconstant for all five of the soils examined. Similar results were obtained for the other two locations, but because of differencesin weather and in the growing season, the threshold time (T25) was found to be dependent on location. The T25 value is alsodependent on surface depressional storage, decreasing with increasing storage. The discovery that water table conditionsbarely satisfying the wetland hydrologic criterion are well correlated to the time required for water table drawdown of 25 cm(T25 values) makes it possible to predict the effects of subsurface drains on wetland hydrology. The lateral effect of a singleditch on wetland hydrology can be computed by using T25 values in solutions to the Boussinesq equation for water tabledrawdown due to drainage to a single drain. While the method was developed for drainage ditches, it may also be used forsubsurface drains.

Modelling two‐dimensional steady‐state groundwater flow and flow sensitivity to boundary conditions in blanket peat complexes

AbstractThis study used a two‐dimensional steady‐state finite‐element groundwater flow model to simulate groundwater flow in two Newfoundland blanket peat complexes and to examine flow system sensitivity to changes in water table recharge and aquifer properties. The modelling results were examined within the context of peat‐forming processes in the two complexes. Modelled flow compared favourably with observed flow. The sensitivity analyses suggested that more highly decomposed bog peat along bog margins probably has/had a positive impact on net peat accumulation within bog interiors. Peat with lower hydraulic conductivity along bog margins effectively impedes lateral drainage, localizes water table drawdown to extreme bog margins, and elevates water tables along bog interiors. Peat formation and elevated water tables in adjacent poor fens/laggs currently rely on placic and ortstein horizons impeding vertical drainage and water flow inputs from adjacent bogs. Modest reductions in atmospheric recharge were found to govern bog‐flow‐system geometries in a way that would adversely affect paludification processes in adjacent fens/laggs. Copyright © 2004 John Wiley & Sons, Ltd.

Effect of water table drawdown on northern peatland methane dynamics: Implications for climate change

As natural sources of methane (CH4), peatlands play an important role in the global carbon cycle. Climate models predict that evapotranspiration will increase under a 2 × CO2 scenario due to increased temperatures leading to lowered water tables at many northern latitudes. Given that the position of the water table within a peatland can have a large effect on CH4 emissions, climate change may alter the CH4 emissions from peatlands in this area. Research was conducted during 2001–2003 on natural and drained (8 years prior) sites within a poor fen in central Quebec. Flux measurements were made for each site at different microtopographical features that varied in depth to water table and vegetation cover. The quantity of CH4 dissolved in the pore water was measured in the field and the potential of the peat for CH4 production and consumption was determined in the laboratory. Methane emissions and storage were lower in the drained fen. Growing season CH4 emissions at the drained site were 55% lower than the control site, primarily due to significantly reduced fluxes from topographic highs (up to 97% reduction), while the flux from topographically low areas remained high. The maintenance of high fluxes at these hollow sites was related to hydrological and ecological effects of the water table drawdown. The removal of standing water removed a potential zone of CH4 oxidation. It also enabled plant colonization at these locations, leading to an increase in gross ecosystem photosynthesis (GEP). At the hollow sites, seasonal CH4 emissions were significantly correlated to seasonal GEP (R2 = 0.85). These results suggest that the response of northern peatland CH4 dynamics to climate change depends on the antecedent moisture conditions of the site. Moreover, ecological succession can play an important role for determining future CH4 emissions, particularly from wetter sites.

Landscape controls on the hydrology of stream riparian zones

Increased knowledge of hydrology is essential to an understanding of the water quality function of stream riparian zones. We examined the effect of upland surficial aquifer size, topography and riparian sediment lithology on the subsurface hydrology of eight riparian sites on glacial till and outwash landscapes in southern Ontario, Canada. Riparian sites had a permanent subsurface hydrologic connection to the adjacent upland in landscapes where upland permeable sediment depths were 2–15 m. In contrast, a hydrologic connection was absent in summer and autumn at riparian sites with <2 m of permeable sediments overlying an aquitard. Riparian zones linked to thicker and more extensive upland aquifers had large relatively constant groundwater inputs that maintained a stable riparian water table. Sites that were seasonally disconnected from uplands had large annual water table drawdowns that dried out the riparian area unless the water table was sustained by the stream. Peats with low hydraulic conductivity and a thinning of highly permeable sediment layers occurred in some riparian zones producing upward groundwater flow that created seeps and surface rivulets. Subsurface flow data indicated a consistent downslope flow path in areas of the riparian zone where the slope gradient was >5%. Where the riparian zone was level to gently sloping (<5%), subsurface flow directions were influenced by stream water level. Riparian subsurface flows at these sites often shifted from a hillslope to stream pattern to a down valley and even a stream to hillslope direction during summers. A conceptual framework of riparian hydrologic types based on hydrogeologic setting is developed to generalize the hydrology of riparian zones at the landscape scale.

Efeitos de inundações temporárias do solo em plantas de ervilha

A cultura de ervilha é uma possível opção para a utilização dos solos de várzea durante o inverno. Porém, esta cultura não está adaptada a condições extremas de falta ou de excesso de água no solo. Nos solos hidromórficos, freqüentemente ocorrem inundações temporárias ou contínuas, afetando diretamente o desenvolvimento da planta. Neste trabalho avaliou-se o efeito da velocidade de rebaixamento do nível freático no desenvolvimento e na produtividade de plantas de ervilha, cv. Axé e determinou-se o estádio de desenvolvimento da cultura com maior sensibilidade ao excesso de água no solo. As análises dos resultados indicam que os efeitos provocados pela inundação do solo e subsequente rebaixamento do nível freático foram proporcionais à duração dos tratamentos, principalmente no estádio vegetativo. A maior suscetibilidade das plantas de ervilha à inundação do solo foi verificada no estádio reprodutivo, com reduções significativas da produtividade em todos os tratamentos. No estádio vegetativo, os rebaixamentos do nível freático em profundidades superiores a 20 cm por dia ocasionaram reduções da produtividade de aproximadamente 20%.

Fine root dynamics in two atlantic white-cedar wetlands with contrasting hydroperiods

Using minirhizotrons, fine root dynamics were compared in two Atlantic white-cedar swamps with contrasting hydroperiods (intermittently and persistently flooded) in order to evaluate the effects of flooding regime on fine root depth, abundance, production, and longevity. Despite the higher mean watertable depth in the persistently flooded, site, no differences in rooting depth were observed. Root length density, root abundance, and root longevity were generally greater in the persistently flooded site throughout the 439-day study period, although root length production was similar between the two sites. These results suggest that prolonged inundation in Atlantic white-cedar swamps decreases root elongation rates, but due to greater observed root densities, stand-level belowground production remains similar to that of intermittently flooded stands. Significant increases in root mortality were observed in the intermittently flooded side during a period of water-table drawdown, suggesting less than optimal hydrologic conditions at this site.

Physiological and morphological response patterns of Populus deltoides to alluvial groundwater pumping.

We examined the physiological and morphological response patterns of plains cottonwood [ Populus deltoides subsp. monilifera (Aiton) Eck.] to acute water stress imposed by groundwater pumping. Between 3 and 27 July 1996, four large pumps were used to withdraw alluvial groundwater from a cottonwood forest along the South Platte River, near Denver, Colorado, USA. The study was designed as a stand-level, split-plot experiment with factorial treatments including two soil types (a gravel soil and a loam topsoil over gravel), two water table drawdown depths ( approximately 0.5 m and >1.0 m), and one water table control (no drawdown) per soil type. Measurements of water table depth, soil water potential (Psi(s)), predawn and midday shoot water potential (Psi(pd) and Psi(md)), and D/H (deuterium/hydrogen) ratios of different water sources were made in each of six 600-m(2) plots prior to, during, and immediately following pumping. Two additional plots were established and measured to examine the extent to which surface irrigation could be used to mitigate the effects of deep drawdown on P. deltoides for each soil type. Recovery of tree water status following pumping was evaluated by measuring stomatal conductance ( g(s)) and xylem water potential (Psi(xp)) on approximately hourly time steps from before dawn to mid-afternoon on 11 August 1996 in watered and unwatered, deep-drawdown plots on gravel soils. P. deltoides responded to abrupt alluvial water table decline with decreased shoot water potential followed by leaf mortality. Psi(pd) and percent leaf loss were significantly related to the magnitude of water table declines. The onset and course of these responses were influenced by short-term variability in surface and ground water levels, acting in concert with physiological and morphological adjustments. Decreases in Psi(pd) corresponded with increases in Psi(md), suggesting shoot water status improved in response to stomatal closure and crown dieback. Crown dieback caused by xylem cavitation likely occurred when Psi(pd) reached -0.4 to -0.8 MPa. The application of surface irrigation allowed trees to maintain favorable water status with little or no apparent cavitation, even in deep-drawdown plots. Two weeks after the partial canopy dieback and cessation of pumping, g(s) and Psi(xp) measurements indicated that water stress persisted in unwatered P. deltoides in deep-drawdown plots.

SUBIRRIGATION SYSTEM WATER TABLE DRAWDOWN ANALYSIS

Subirrigation, a practice that combines irrigation with drainage, is used to maintain a shallow water table at anoptimum depth for increased crop yield. Rainfall events, while subirrigating, cause the water table to rise and saturate allor part of the crop root system. Damage to the crop can occur if the water table remains within the root zone for an extendedtime. A mathematical water table drawdown simulation model for a subirrigation system using a weir was developed. Themodel was used to simulate the time required for the water table to recede to the beforerain level after a rainfall event raisesthe midpoint water table to the soil surface, with and without the weir. Removing the weir after rainfall events reduces thetime required for the water table to recede. The model shows the need to remove the weir to prevent crop damage for rainevents that saturate the soil profile to near the field surface.

Hydrologic and chemical control of Phragmites growth in tidal marshes of SW Connecticut, USA

We compared hydrology and porewater chemistry along transects in 3 tidal marshes vegetated by Spartina alterniflora and by the invasive species Phragmites australis. Relative to the Phragmites zone, the S. alterniflora zone occurred at lower tidal elevations in all 3 marshes and was characterized by greater depth of flooding and shorter periods of water-table drawdown below the soil surface. Penetration by Phragmites into the S. alterniflora zone appeared to be limited by exten- sive soil saturation at the leading edge of Phragmites growth. This mixed-species zone occurred at the intersection of the low-tide groundwater table with the soil surface and was characterized by sig- nificantly higher concentrations of porewater sulfide (739 ± 63 µM) relative to the monospecific Phragmites zone (335 ± 63 µM). Dissolved ammonium and phosphate concentrations were also sig- nificantly lower in the Phragmites zone relative to S. alterniflora and mixed-species zones of vegeta- tion. Discriminant analysis of the measured hydrologic and chemical variables identified depth of flooding, flooding frequency and duration, and sulfide concentration, as the most significant vari- ables distinguishing Phragmites stands along the estuarine salinity gradient. Together, these results indicate a greater flooding tolerance by Phragmites in marshes where porewater sulfide and salinity are lower. From a hydrologic and chemical perspective, opportunities for Phragmites invasion and expansion should be greater in oligohaline and mesohaline tidal marshes relative to polyhaline habitats. To control the growth of Phragmites in tidal marshes of management concern, both the feasibility and need for methods that increase flooding depth, frequency, salinity and/or sulfide concentrations should be considered.

Interannual Variability in Net Ecosystem CO2 Exchange at the Arctic Treeline

Net ecosystem exchange (NEE) of carbon dioxide was measured at treeline forest and fen tundra sites near Churchill, Canada during three consecutive growing seasons (1997–1999). Although both sites demonstrated a net uptake of CO2 in each of the years, there was considerable variation in the flux between sites and between years. Mean daily NEE at the forest varied from –1.5 (±0.25 SD) g CO2 m−2 d−1 in 1997 to –7.3 (±0.39) g CO2 m−2 d−1 in 1999. The fen mean daily NEE varied from –1.1 (±0.36) g CO2 m−2 d−1 in 1997 to –3.1 (±0.50) g CO2 m−2 d−1 in 1998. Integrated over the 65-d measurement period the forest was a net sink of –100, –313, and –478 g CO2 m−2 in 1997, 1998, and 1999, respectively, and the fen net sink was –73, –202, and –38 g CO2 m−2, respectively. Interannual variations in the fen NEE were closely related to water table drawdown (soil wetness), the forest NEE was unaffected by water table change. The forest seasonally cumulative NEE was closely associated with timing of snowmelt and accumulated heat content prior to leaf-out. Earlier snowmelt and greater heat accumulation produced a larger growing season sink. It is likely these events also influenced the fen NEE, but the relationship is complicated by water table effects. These results suggest that carbon dioxide exchange varies considerably across the northern treeline, and that there may be an important biospheric feedback between climate warming, treeline advance, and carbon cycling.

Interannual Variability in Net Ecosystem CO 2 Exchange at the Arctic Treeline

Net ecosystem exchange (NEE) of carbon dioxide was measured at treeline forest and fen tundra sites near Churchill, Canada during three consecutive growing seasons (1997–1999). Although both sites demonstrated a net uptake of CO2 in each of the years, there was considerable variation in the flux between sites and between years. Mean daily NEE at the forest varied from –1.5 (±0.25 SD) g CO2 m−2 d−1 in 1997 to –7.3 (±0.39) g CO2 m−2 d−1 in 1999. The fen mean daily NEE varied from –1.1 (±0.36) g CO2 m−2 d−1 in 1997 to –3.1 (±0.50) g CO2 m−2 d−1 in 1998. Integrated over the 65-d measurement period the forest was a net sink of –100, –313, and –478 g CO2 m−2 in 1997, 1998, and 1999, respectively, and the fen net sink was –73, –202, and –38 g CO2 m−2, respectively. Interannual variations in the fen NEE were closely related to water table drawdown (soil wetness), the forest NEE was unaffected by water table change. The forest seasonally cumulative NEE was closely associated with timing of snowmelt and accumulated heat content prior to leaf-out. Earlier snowmelt and greater heat accumulation produced a larger growing season sink. It is likely these events also influenced the fen NEE, but the relationship is complicated by water table effects. These results suggest that carbon dioxide exchange varies considerably across the northern treeline, and that there may be an important biospheric feedback between climate warming, treeline advance, and carbon cycling.

One last visit to the capillarity correction for free surface flow

When a capillarity correction is incorporated into the diffusion equation to describe the sudden drawdown of a water table, the resulting differential equation preserves, instead of smoothes, irregularities in the initial data. We describe how this problem can be handled by a proper and careful interpretation of the initial conditions, explain why it works, and illustrate the resulting behavior with numerical experiments.

Monitoring of drawdown pattern during pumping in an unconfined physical aquifer model

AbstractIn this study, we attempted to analyse a drawdown pattern around a pumping well in an unconfined sandy gravelly aquifer constructed in a laboratory tank by means of both experimental and numerical modelling of groundwater flow. The physical model consisted of recharge, aquifer and discharge zones. Permeability and specific yield of the aquifer material were determined by Dupuit approximation under steady‐state flow and stepwise gravitational drainage of groundwater, respectively. The drawdown of water table in pumping and neighbouring observation wells was monitored to investigate the effect of no‐flow boundary on the drawdown pattern during pumping for three different boundary conditions: (i) no recharge and no discharge with four no‐flow boundaries (Case 1); (ii) no recharge and reservoir with three no‐flow boundaries (Case 2); (iii) recharge and discharge with two no‐flow boundaries (Case 3). Based on the aquifer parameters, numerical modelling was also performed to compare the simulated drawdown with that observed. Results showed that a large difference existed between the simulated drawdown and that observed in wells for all cases. The reason for the difference could be explained by the formation of a curvilinear type water table between wells rather than a linear one due to a delayed response of water table in the capillary fringe. This phenomenon was also investigated from a mass balance study on the pumping volume. The curvilinear type of water table was further evidenced by measurement of water contents at several positions in the aquifer between wells using time domain reflectometry (TDR). This indicates that the existing groundwater flow model applicable to an unconfined aquifer lacks the capacity to describe a slow response of water table in the aquifer and care should be taken in the interpretation of water table formation in the aquifer during pumping. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Sulphate Mobilization and Pore Water Chemistry in Relation to Groundwater Hydrology and Summer Drought in two Conifer Swamps on the Canadian Shield

Variations in sulphate (SO42-) concentration of porewater and net SO42- mobilization were related to differences in water level fluctuations during wet and dry summers in two conifer swamps located in catchments which differed in till depth and seasonality of groundwater flow. Sulphate depletion at the surface and in 20 cm porewater coincided with anoxia and occurred mainly during the summer when water levels were near the peat surface and water flow rates were low in both catchments. There was an inverse relationship between net SO42- mobilization and water level elevation relative to the peat surface, explaining variation in SO42- dynamics between the swamps during summer drought periods. Aeration of peat to 40 cm and a large net SO42- mobilization (10–70 mg SO42- m-2 d-1) occurred during a dry summer in which the water level dropped to 60 cm below the surface in the swamp receiving ephemeral groundwater inputs from shallow tills within the catchment. This resulted in high SO42- concentrations in the surface water and porewater (30–50 mg L-1), and elevated SO42- concentrations remained through the fall and winter. In contrast, within the swamp located in the catchment with greater till depth (> 1 m), continuous groundwater inputs maintained surface saturation during the dry summer, and SO42- mobilization and concentrations of SO42- in the pore water during the following fall did not increase. Susceptibility to large water table drawdown and mobilization of accumulated SO42- is influenced by the occurrence of ephemeral vs. continuous groundwater inputs to valley swamps during dry summer periods in the Canadian Shield landscape. This study reveals that extrapolation of results of SO42- cycling from one wetland to another requires knowledge of the hydrogeology of the catchment in which the wetlands are located.

Methane Emissions from a Gully Mire in Mid-Wales, U.K. under Consecutive Summer Water Table Drawdown

Wetlands are a major contributor to the global methane (CH{sub 4}) budget. Currently there is a consensus view that drought restrains CH{sub 4} emissions from wetlands. However, a 6-year field experiment investigating the effects of persistent summer water table drawdown on a gully mire in mid-Wales, U.K. demonstrated a 2-year boom and bust cycle of CH{sub 4} emissions followed by a recovery during the third year, when compared with emissions from a control wetland in which the water table remained at or above the peat surface. This cyclical trend was subsequently replicated by the control wetland following natural summer drought. The experimental wetland was affected less by this natural drought following previous water table drawdown. Depth profiles of CH{sub 4} production suggested that consecutive water table drawdown resulted in a translocation of peak methanogen activity lower down the peat profile, where emissions were affected less by subsequent drawdown. These findings suggest that wetland CH{sub 4} flux models need to consider the possibility that methane emission-water table relationships are dependent on the pattern and frequency of drought events.

SULPHATE DYNAMICS IN RELATION TO GROUNDWATER–SURFACE WATER INTERACTIONS IN HEADWATER WETLANDS OF THE SOUTHERN CANADIAN SHIELD

The spatial and temporal distribution of sulphate (SO4) concentrations in peat pore water and the outlet streams of two forested swamps was related to variations in the magnitude of upland runoff, wetland water levels and flow path. The swamps were located in headwater catchments with contrasting till depths typical of the southern Canadian Shield. Inputs of SO4 from shallow hillslope tills and streams showed little seasonal variation in either source or concentration in both swamps. Sulphate dynamics at the outlet stream reflected hydrological and biogeochemical processes within the valley wetlands, which in turn were partly controlled by catchment hydrogeology. During high runoff, maximum water table elevations and peak surface flow in the swamps resulted in upland inputs largely bypassing anoxic peat. Consequently, SO4 concentrations of 8–10 mg/l at the swamp outlets were similar to stream and groundwater inputs. During periods of low flow, concentrations of SO4 at the swamp outlets declined to less than 3 mg/l. At this time lower water table elevations resulted in increased interaction of input water with anoxic peats, and therefore, SO4 reduction. Contrasts in till depth and the nature of groundwater flow between catchments resulted in differences in SO4 dynamics between years and swamps. In dry summers the absence of groundwater inputs to the swamp in the catchment with thin till resulted in a large water table drawdown and re-oxidation of accumulated S, which contributed to maximum SO4 concentrations (up to 35 mg/l) during storm runoff. Continuous groundwater input to the swamp in the catchment with deeper till was critical to maintaining saturated surfaces and efficient SO4 retention during both dry and wet summers. A conceptual model of wetland SO4 retention and export, based on catchment hydrogeology, is developed to generalize the SO4 dynamics of valley bottom wetlands at the landscape scale. © 1997 by John Wiley & Sons, Ltd.

EFFECT OF SPATIAL VARIABILITY IN HYDRAULIC CONDUCTIVITY ON WATER TABLE DRAWDOWN

The field variability in saturated hydraulic conductivity and its impact on water table drawdown wasinvestigated by numerically solving the Boussinesq equation for 2-D, transient water flow between parallel drains. Over500 Monte-Carlo simulations were used to carry-out a parametric uncertainty analysis and determine the risk ofwaterlogging of the crop root zone from a given drainage system. The rate of water table drawdown between thedeterministic and stochastic solutions was compared and the growth in uncertainty in the stochastic solution with timewas investigated. Results indicate that the geometric mean of K values can be used in the deterministic design of drainagesystems. However, a deterministic approach using the arithmetic mean of K values may underdesign a drainage system incontrast with the stochastic solution which provides a lesser water table drawdown. In general, the proposed method canbe used to design drainage systems to meet a specific drawdown criteria with a specified degree of success.

FLOW REVERSALS IN PEATLANDS INFLUENCED BY LOCAL GROUNDWATER SYSTEMS

Piezometric head data from various depths were examined at two peatlands in Ontario, Canada and one peatland in Sweden influenced by small-scale, shallow groundwater systems. Data from different hydrogeological settings show reversals in groundwater flow leading to discharge in topographically high regions of peatlands in isolation from large-scale groundwater flow. It is suggested that subsurface flow within peat can reverse in direction in response to water deficit and water-table drawdown. The data presented here refute the assumption that local groundwater flow in peatlands is unidirectional and further illustrate the fact that measurable subsurface water flow can occur at depth in peat isolated from large-scale groundwater flow systems. In the light of implicit assumptions made by many workers on water movement in peatlands, especially when connected to small-scale groundwater systems, the consequences of such reversals are paramount in understanding the hydrology and biogeochemistry of peatlands. © 1997 by John Wiley & Sons, Ltd.

Channels and valleys on Mars: cold climate features formed as a result of a thickening cryosphere

Large flood channels, valley networks, and a variety of features attributed to the action of ground ice indicate that Mars emerged from heavy bombardment around 3.8 Gyr ago, with an inventory of water at the surface equivalent to at least a few hundred meters spread over the whole planet, as compared with 3 km for the Earth. The surface water resided primarily in a porous, kilometers thick, megaregolith created by the high impact rates. At the end of heavy bombardment a rapid decline in erosion rates by a factor of 1000 suggests a major change in the global climate. It is proposed that at this time the elimate became similar to today's and that this climate has been maintained throughout the rest of Mars' history. The various drainage features represent an adjustment of the distribution of water to the surface relief inherited from the period of heavy bombardment and to a thickening of the cryosphere as the heat flow declined. The valley networks formed mostly at the end of heavy bombardment when erosion rates were high and climatic conditions permitted an active water cycle. They continued to form after heavy bombardment when the cryosphere started to form by a combination of episodie flooding and mass-wasting aided by the presence of liquid water at shallow depths. As the cryosphere thickened with declining heat flow, water could no longer easily access the surface and the rate of valley formation declined. Hydrostatic pressures built below the cryosphere. Eruptions of groundwater became more catastrophic and massive floods resulted, mainly in upper Hesperian time. Flood sources were preferentially located in low-lying, low-latitude areas where the cryosphere was thin, or near volcanoes where a thinner than typical cryosphere is also expected. Floods caused a drawdown in the global water table so that few formed in the second half of Mars' history. The floodwaters pooled in low-lying areas, mostly in the northern plains. Some of the water may still be present as thick ice deposits, some has been lost to space, particularly during periods of high obliquity.