Hydrochemistry, weathering and weathering rates on Madeira island

Abstract

Madeira island consists of Miocene to Pleistocene lavas and pyroclasts. Major rock types are alkali-basalts, basanites and hawaiites; principal soil types are leptosols, andosols and cambisols. Our main objective was to link the chemistry of ground waters to weathering reactions and rates. We collected 40 shallow groundwater samples, remote from human activities. With a few exceptions, the ranges of electrical conductivities were 29 -176 mS/cm and of pH 5.8-8.5. The calculated P CO 2 was generally higher than the atmospheric value. The contribution of sea salt to the water chemistry was 30^9%. Corrected for sea salt, the cation concentrations (in meq/l) decrease in the order CaThe concentrations of SO 4 22 and NO 3 2 are very low. We calculated that the total annual chemical denudation rate in the studied area amounts to 37^12 g/m 2 , consuming 0.86^0.38 mol CO 2 /m 2 . To achieve our main objective, a set of mole balance equations-ðAX ¼ BÞ-was used, where A is a composite matrix of coefficients, including ratios between stoichiometric coefficients as determined by the weathering reactions and coefficients accounting for unconstrained contributions, B is the vector with a water composition, and X is the set of mole fractions of dissolved primary minerals plus the residual concentrations of the unconstrained contributions. Olivine (Ol), pyroxene (Py) and plagioclase (Pl) were considered to be the major primary minerals, and smectite, vermiculite, halloysite, allophane, gibbsite and hematite the secondary minerals in the weathering reactions. Using iterative procedures, whereby mixtures of secondary products as well as the composition of plagioclase are allowed to change, we selected one best-fit set of weathering reactions for each spring by checking all possible solutions of the mole balances against predefined boundary conditions. At odds with Goldich (1938) sequence, our model results indicate-for most best -fit sets-a weathering rate sequence of Pl . Ol . Py, but such reverse order is not unique. On average, the annual weathering rates (in mol/(ha y vol% mineral)) are 44^19 (Pl), 29^14 (Ol) and 22^13 (Py). q

Groundwater Recharge Potential for Sustainable Water Use in Urban Areas of the Jequitiba River Basin, Brazil

“…Direct methods comprise geological and geophysical explorations, gravimetric and magnetic methods, and drilling tests. Indirect methods include hydrological modeling [58][59][60][61][62][63][64][65][66] using geographic information systems (GIS) combined with fieldwork, geochemical tracers [67][68][69], survey of specialized literature for standard values [15,16,70,71], among others. The choice for a method (direct or indirect) should consider the precision level needed, the project execution, and the resources available.…”

Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA

“…Warm, arid and semiarid soils include secondary minerals comprising crystalline Fe-oxyhydroxides and phyllosilicates that grade from dioctahedral smectite to mixed-layer smectite-kaolin and kaolinite or dehydrated/hydrated halloysite with increasing annual rainfall (Singer, 1966;Graham and Francovizcaino, 1992;Righi et al, 1999;Dekayir and El-Maataoui, 2002;Singer et al, 2004;Mirabella et al, 2005;Abayneh et al, 2006;Barbera et al, 2008;Egli et al, 2008). Warm, wet temperate and tropical soils generally exhibit secondary mineral fractions dominated by halloysite, hydroxy-Al interlayered smectite, gibbsite and crystalline Fe-oxyhydroxides because of high rates of desilication and generally moist soil conditions that favor hydrated kaolin phases (Kautz and Ryan, 2003;Van der Weijden and Pacheco, 2003;Ndayiragije and Delvaux, 2004;Kleber et al, 2007;Zhang et al, 2007). In contrast, cool pedogenic environments exhibit secondary minerals dominated by SRO phases that include allophanic materials, ferrihydrite, and organo-metal complexes (Shoji et al, 1993;Chadwick et al, 2003;Pokrovsky et al, 2005).…”

Clay minerals, oxyhydroxide formation, element leaching and humus development in volcanic soils

“…The subtropical and rather dry climate in Lipari was the probable cause of the very slow mineral reaction rates. The chemical weathering of basalt is, however, known to be more rapid and could, therefore, be an important sink in the global CO 2 -cycle ( Van der Weijden and Pacheco, 2003). Information about mid-to long-term weathering rates is rather rare and often not unequivocal.…”