Zone Of Water Table Fluctuation Research Articles

EDGE EFFECTS IN GEOMORPHOLOGY

Edge effects in geomorphology produce features or processes along the boundaries between landscape elements, which are distinctly different from those of the adjacent elements. These effects are unique to the edge environment, as opposed to simply being transitional in nature. Three general types of edge effects are identified and illustrated with examples from the North Carolina coastal plain. First, ecological edge effects may have a geomorphic expression owing to the biogeomorphic impacts of plants and animals. This is illustrated by ant bioturbation along a forest-grass boundary. A concentration of ant mounding in the edge zone affects bioturbation rates, soil turnover times, and surface physical properties that are distinct from those in the adjacent forest or grass. Second, edge effects may be associated with unique geomorphic controls along boundaries. In the coastal plain, for example, ferricretes form along valley-side slopes where the discharge of iron-enriched groundwater coincides with erosion or mass wasting, which exposes the zone of water-table fluctuation. Third, geomorphic edge effects may be associated with force-resistance differences along landscape boundaries. This phenomenon is shown to be responsible for the development of eolian ridges along some forest cropland boundaries. It is important to recognize that landforms and processes along edges may not be representative of those in the adjacent landscapes. [Key words: edge effects, geomorphology, North Carolina.]

The longevity of minewater pollution: a basis for decision-making

Pollution from abandoned mines is a serious, widespread and increasingly common cause of surface water degradation in the LOIS study region. Indeed, in some areas of the UK it is now the single greatest cause of freshwater pollution. Remediation of such pollution in the long-term is probably best achieved by passive treatment, which has the advantage of concentrating expenditure in capital costs, with only modest financial commitments for long-term maintenance. To plan such remediation, however, it is important to have an understanding of the physical and chemical processes governing pollutant release over scales of many decades. By analysing water quality records for major abandoned mine discharges in Scotland, Wales and Cornwall, long-term acidity generation is shown to have two components: ‘vestigial’ and ‘juvenile’. The vestigial component relates to the geochemical trauma which occurs as abandoned mine voids are allowed to fill with water for the first time, taking ferrous and ferric hydroxysulphate salts (intermediate products of pyrite oxidation) into solution. Dissipation of vestigial acidity is primarily controlled by the hydraulic retention time of the flooded mine system, and will generally be accomplished in less than 40 years. Juvenile acidity arises from on-going pyrite oxidation in the zone of water table fluctuation within the mined system, and can be expected to continue for many hundreds of years, until the supply of pyrite is finally exhausted. Rational planning for the remediation of minewater pollution should be based on intensive treatment of discharges while the vestigial acidity is being depleted, followed by long-term passive treatment of the juvenile acidity.

Separating the Effects of Litter Quality and Microenvironment on Decomposition Rates in a Patterned Peatland

Decomposition rates, measured as proportion of original ash-free dry mass lost from litter bags, were studied on four microhabitats of an ombrogenous peatland in southwestern Scotland: a Racomitrium lanuginosum hummock (HR), a Sphagnum capillifolium hummock (HS), a Sphagnum papillosum lawn (L), and a Sphagnum cuspidatum hollow (H). Reciprocal transplant experiments, in which litter bags were swapped among depths both within and among microhabitat types, separated the effects of litter quality (litter type and degree of humification of the peat) and microenvironment (water table position and microhabitat type). All were important determinants of mass loss. Decomposability of the litter from different microhabitats increased in the order HR < HS < L < H. Chemical 'ageing' of the peat reduced rates of decay in highly humified peat, although a history of decay was associated with maximum decomposability of peat from HR hummocks. The suitability of hollows for decay was significantly less than for HR and HS hummocks and lawns. Peat lost mass most slowly when placed below the lowest water table, but the highest mass losses were for peat placed in, or slightly above, the zone of water table fluctuation. Mass loss decreased with depth for peat decaying in its natural position in hollows and lawns and the oxic layer of HS hummocks. A peak in mass loss occurred within the zone of water table fluctuation in HS hummocks, and just above the highest water table in HR hummocks. The results support earlier suggestions that differences due to chemical ageing of peat contribute to differences in decomposition rates between hummocks and hollows, and that hummock species are intrinsically more resistant to decay than hollow species. The pattern was complicated further, however, by the effects of water table position and microhabitat type. The combined effects of litter quality (species composition and degree of humification of the litter) and environment (microhabitat of incubation and position of incubation relative to water table) are complex, and these complexities must be incorporated into models of decomposition.

Elemental Distribution and Pyrite Occurrence in a Freshwater Peatland, Alberta

A continental minerotrophic peatland in Alberta was studied for its botanical, petrographical, mineralogical and geochemical characteristics. Changes in peat stratigraphy and mineralogy are related, and development toward oligotrophic conditions is recorded. Elemental concentration and distribution vary in the peat profile, reflecting the influence of natural and anthropogenic factors on peat geochemistry. Many elements (Al, As, Ba, Co, Cr, Fe, Mn, Th, V) accumulate in the zone of water table fluctuation as a result of changes in redox chemistry. The relative elemental concentration at the surface and near-surface (e.g., As, Cr, Sb, Th, Ti, V) may also be attributed to atmospheric deposition related to anthropogenic activity. Chemical analysis (Al, Fe, Mg, Na, Si, Ti) also reveals the presence of a non-visible volcanic-ash-rich layer near the base of the peat profile, possibly Mazama tephra. This study provides evidence of pyrite formation in a freshwater peat-forming environment, associated with high-sulphur peats.

Distribution of Carbonate Cements in Quaternary Alluvial-Fan Deposits, Birch Creek Valley, East-Central Idaho--Diagenetic Model: ABSTRACT

Quaternary alluvial-fan deposits in Birch Creek Valley are poorly sorted carbonate gravels that have undergone diagenesis in the meteoric realm through the dissolution and precipitation of calcium carbonate. Three diagenetic zones are documented on the basis of cement morphologies and paragenesis: (1) near-surface vadose, (2) vadose, and (3) vadose-phreatic. Cements formed in the near-surface vadose zone result from both pedogenic and nonpedogenic processes. Pedogenic processes predominate within the upper meter of fan surfaces, whereas nonpedogenic processes cause case-hardening on steep, unvegetated outcrops. Pedogenic cementation proceeds in a series of four morphologic stages and is characterized by clotted micrite and fibrous sparry calcite, commonly with gravitational morphologies and intricate banding. Nonpedogenic cements are primarily micritic to finely crystalline with homogeneous or clotted textures; microdigitate cements are common on the undersides of clasts. Dissolution and incipient cementation are typical in the vadose zone; cements are best developed beneath large clasts. Thin, banded, gravitational cement, grain-contact cement, rare syntaxial overgrowths, and the lack of clotted micrite are indicative of vadose cementation. Well-cemented fanglomerate reflects progressive cementation in the vadose-phreatic zone, or in a zone of water-table fluctuation. Two generations of cement are generally apparent. Early micrite cement forms discontinuous to continuous rims and is followed by an isopachous sparry cement. Syntaxial overgrowths are relatively common on monocrystalline grains. The degree of cementation is variable and appears to be related to grain size, sorting, and packing geometries. The distribution and nature of the cements suggest that cementation is initiated soon after deposition and proceeds simultaneously in each diagenetic zone. End_of_Article - Last_Page 454------------

Peat hydrology in the Hula Basin, Israel: I. Properties of peat

The results of laboratory and field studies concerned with various mechanical and hydraulic properties of peat in the Hula Basin are presented. Particular emphasis has been placed on the measurement of bulk density, porosity and void ratio, hydraulic conductivity, water retention under unsaturated conditions, compressibility, release of water due to consolidation and the content of organic matter. It is shown that the properties of peat in the basin change drastically when it becomes partially desaturated. As a result of this change, one can distinguish between a permanently saturated layer below the zone of water-table fluctuation having relatively uniform characteristics and an overlying partially desaturated layer the properties of which vary gradually with depth. The saturated layer is currently undergoing a process of consolidation and its properties may therefore be expected to change in the future. One of the conclusions reached in this work is that compressibility is very important and must not be neglected in dealing with fluid flow through peats. This is true for confined as well as unconfined (i.e., water-table) conditions.

Water-Table Effects on Seacoasts

Calcareous cementation of unconsolidated sediments and soils within the zone of water-table fluctuation in tropical climates results in the development of widespread layers of water-table rock that extend between beach rock and stream rock outcrops as well as for considerable distances inland. Along cliffy coasts in any climate, the water table separates weathered and leached rock, which is readily erodible, from underlying sections of relatively resistant rock. Along seacoasts, the water-table boundary commonly is expressed by the presence of platforms, shoals, and erosionally flattened bedrock in beaches (pseudo-beach rock). For the reason that coastal retreat lowers water tables, truncated flats commonly develop at successively lower levels, and the resulting topography is subject to misinterpretation, such as the postulation of a very recent, somewhat higher, stand of sea level.

Alteration of ilmenite

Ilmenite and its alteration products in commercial heavy-mineral concentrates were studied by means of chemical analysis, optical microscopy, electron microscopy, electron microprobe, and X-ray diffraction. Breakdown of ilmenite involves oxidation and ultimate removal of iron. Alteration starts on surface of grains and on cleavages and fractures. Phases present in the altered material are ilmenite, rutile, and a new iron-titanium oxide named pseudorutile. Final removal of iron from altered ilmenite occurs only in or above the zone of water-table fluctuation.