A statistical assessment of the impact of land uses on surface water quality indexes.

The release of wastewater from various land uses is threatening the quality of surface water. Different land uses pose varying degrees of danger to water resources. The hazardous extent of each activity depends on the amount and characteristics of the wastewater. The concept of the contamination potential index (CPI) of an activity is introduced and applied here. The index depends on the quantity of wastewater from a single source and on various chemicals in the waste whose concentrations are above allowable standards. The CPI concept and the land use impact assessment are applied to the surface water conditions in Nakhon Nayok Province in the central region of Thailand. The land uses considered in this study are residential area, industrial zone, in-season and off-season rice farming, and swine and poultry livestock. Multiple linear regression analysis determines the impact of the CPIs of these land uses on certain water quality characteristics, i.e., total dissolved solids, electrical conductivity, phosphate, and chloride concentrations, using CPIs and previous water quality measurements. The models are further verified according to the current CPIs and measured concentrations. The results of the backward and forward modeling show that the land uses that affect water quality are off-season rice farming, raising poultry, and residential activity. They demonstrate that total dissolved solids and conductivity are reasonable parameters to apply in the land use assessment.

Upscaling heterogeneity in aquifer reactivity via the exposure-time concept: inverse model.

A novel inverse technique is proposed to quantitatively characterize macroscopic variability in aquifer reactivity in a Lagrangian representation. Reactivity heterogeneity is expressed in terms of distributions of flux over cumulative time of exposure of the solution to reactive surface area, termed here 'cumulative reactivity'. In cases involving single aqueous species the combined effects of physical and reactivity heterogeneity on reactive solute transport can often be established and further investigated through joint distributions of flux over travel time and cumulative reactivity. The inverse technique requires the breakthrough curve of a passive tracer to determine the distribution of flux over travel time, and additional breakthrough curves of reactive tracers provide additional moments of the distribution of flux over cumulative reactivity given travel time. Thus breakthroughs of one passive and two reactive tracers can provide the mean and variance of the distribution of flux over cumulative reactivity. This Lagrangian characterization is achieved with knowledge of the types of reactive surfaces present, but not their spatial locations. The distributions can subsequently be applied via forward modeling using the same technique to predict breakthrough curves of other solutes undergoing first-order reactions in similar physically and chemically heterogeneous configurations.

Upscaling heterogeneity in aquifer reactivity via exposure-time concept: forward model.

Reactive properties of aquifer solid phase materials play an important role in solute fate and transport in the natural subsurface on time scales ranging from years in contaminant remediation to millennia in dynamics of aqueous geochemistry. Quantitative tools for dealing with the impact of natural heterogeneity in solid phase reactivity on solute fate and transport are limited. Here we describe the use of a structural variable to keep track of solute flux exposure to reactive surfaces. With this approach, we develop a non-reactive tracer model that is useful for determining the signature of multi-scale reactive solid heterogeneity in terms of solute flux distributions at the field scale, given realizations of three-dimensional reactive site density fields. First, a governing Eulerian equation for the non-reactive tracer model is determined by an upscaling technique in which it is found that the exposure time of solution to reactive surface areas evolves via both a macroscopic velocity and a macroscopic dispersion in the artificial dimension of exposure time. Second, we focus on the Lagrangian approach in the context of a streamtube ensemble and demonstrate the use of the distribution of solute flux over the exposure time dimension in modeling two-dimensional transport of a solute undergoing simplified linear reversible reactions, in hypothetical conditions following prior laboratory experiments. The distribution of solute flux over exposure time in a given case is a signature of the impact of heterogeneous aquifer reactivity coupled with a particular physical heterogeneity, boundary conditions, and hydraulic gradient. Rigorous application of this approach in a simulation sense is limited here to linear kinetically controlled reactions.