Weathering water chemistry

Effect of altitude of catchment area on water chemistry and weathering rates (Data from Zobrist and Drever, 1990)... 

Daccord G, Lenormand R (1987) Fractal patterns from chemical dissolution. Nature 325 41 3 Daccord G, Lietard O, Lenormand R (1993) Chemical dissolution of a porous medium by a reactive fluid, 2, Convection vs. reaction behavior diagram. Chem Eng Sci 48 179-186 Darmody RG, Thorn CE, Harder RL, Schlyter JPL, Dixon JC (2000) Weathering implications of water chemistry in an arctic-alpine environment, north Sweden. Geomorphology 34 89-100 Dijk P, Berkowitz B (1998) Precipitation and dissolution of reactive solutes in fractures. Water Resour Res 34 457-470... 

The primary objectives of mass-balance studies are (i) quantify the mass fluxes into and out of watershed systems (ii) interpret the reactions and processes occurring in the watershed that cause the observed changes in composition and flux (iii) determine weathering rates of the various minerals constituting the bedrock, regolith, and soils of the watershed and (iv) evaluate which mineral phases are critically involved in controlling water chemistry to help develop models of more general applicability (i.e., transfer value). 

Chemical weathering and lithologic controls of water chemistry in a high-elevation river system Clark s fork of the Yellowstone River, Wyoming and Montana. Water Resour. Res. 35, 1643-1656. 

Elevated pH values occur in waters whose chemistry is dominated by minerals, most of which, as noted above, are salts of strong bases and weak acids. Thus, in the absence of sources of acidity, mineral carbonate and silicate and alumino-silicate minerals tend to raise the pH to values of 9 to 10 or even higher. Such high pH s are found in arid soils and in some deep groundwaters that are not exposed to fresh recharge and so can be said to be rock dominated as opposed to water dominated. In such systems the minerals present can exist stably, without weathering. Water pH values above 10 are exceptional and may reflect contamination by strong bases, such as NaOH or Ca(OH)2. 

Fig. 12.20 Results of a steady-state simulation with a coupled model for ocean circulation, water chemistry and sediment diagenesis. Major control parameters and forcings comprise a large-scale geostrophic flow field, primary productivity controlled by nutrient advection, export production and sediment accumulation, as well as CO input by weathering and CO -exchange with the atmosphere, a) Export production (mol m yr ), b) CaCO export production (both mol m yr ), c) wt% CaCOj, d) CaCO mass accumulation rate (g cm kyr ) (from Archer et al. 1998).

One factor that has been linked with long-term changes in the chemistry of surface waters and soils is the influence of variations in weather, particularly temperature and precipitation. The variability that fluctuations in weather induce in surface water chemistry (Hindar et al. 2003) may obscure the detection of trends resulting from reductions in SO/ deposition. On the other hand, use of long-term data may obscure shorter trends occurring within the series, particularly if the trends are not uni-directional. In this case, the detection of trends may be dependent on the time window used in the analysis, and can affect both the detection of the presence and the direction of a trend (Clair et al. 2002). For example, Clair et al. (1992) found significant decreases in SO/ concentrations in lakes in Nova Scotia, Canada, for the period 1983-1989, but with the addition of two more years of data, many of these trends were reversed (Clair et al. 1995). 

Recently, detailed studies on the factors controlling river water chemistry such as biological activity (Likens et al. 1987), soilwater (Holland 1978 Likens et al. 1987) and geography (Drever 1988) have been done. However, the chemical compositions of major river water plot in chemical weathering-dominated region. 

Therefore, the relationship between chemical weathering and river water chemistry is considered below. 

Gaillardet et al. (1999) estimated contribution of chemical weathering to river water chemistry based on Sr isotopic composition, concentration ratio of elements to Na, and runoff. According to their works, contributions of chemical weathering for Si and K are 100 % and 60 %, respectively. Those for Na, Ca, HCOa" and Sr are less than 50 %. It is obtained that the contribution of Si is the highest. Therefore, Si concentration of river water is considered below. 

It is clear that the detailed soil and water chemistry of the region and the stable isotope systematics provide indicators of the agricultural and mineral resource potential of this vast region and the climate history that has influenced the Amazon Basin. Recent data also shows that the chemistiy and physical state and complexity of the weathered materials can only be explained by consideration of specific processes involving microorganisms. From surface to depths of lOO s of meters, the weathered rocks are porous and permeable and obviously host living organisms. Modem observations show that the biosphere can extend to depths of several kilometers. 

---