Using TDR and Inverse Modeling to Characterize Solute Transport in a Layered Agricultural Volcanic Soil

this study was reported by Regalado et al. (2003) and Muñoz-Carpena et al. (2005). Volcanic soils exhibit particular physical-chemical properties (i.e., The intensive use of agrichemicals in volcanic agriculstrong and stable natural aggregation and high content of variablecharge minerals) that may influence solute transport. To determine tural fields, as for many other areas of the world, has led if such techniques like TDR and inverse modeling are useful for to groundwater contamination. Especially in Tenerife analyzing solute transport in volcanic soils, we studied the governing (Canary Islands), widespread groundwater pollution has transport processes by means of a miscible displacement experiment been reported in the traditional agricultural areas along of Br! in a large undisturbed soil monolith. Bromide resident conthe coast (Muñoz-Carpena et al., 2001). This is a concentrations at several depths were monitored successfully with TDR tinuing problem that requires the minimization of agritechnology, while parameters for the convective–dispersive (CDE) chemical leaching losses. In this context, field-tested and mobile–immobile (MIM) transport models were estimated by numerical leaching models are recommended for studyinverse modeling. For the relatively high soil moisture conditions, ing solute transport through the vadose zone to the undertypical of high frequency-irrigation systems that we considered, Br! lying groundwater. Such models can be useful tools for was found to move slowly by convection–dispersion. Simulations with the CDE and MIM transport models yielded very similar results. Alunderstanding the movement of solutes in the soil and though Br! is generally assumed to behave as a tracer, we found that for evaluating the potential effect of alternative agriculthis anion in our experiment was subject to adsorption at the bottom tural practices on groundwater contamination. Performpart of the monolith. This may be explained by the variable-charge ing a representative study of the solute transport requires nature of the minerals (Fe and Al oxihydroxides) present in this volthe application of the simulation model at the field scale, canic soil, which exhibited anion exchange when the pH of the soil or at the scale of large undisturbed soil cores (monoliths). solution decreased below the zero point of charge. The use of numerical simulation models further requires the estimation of vadose zone flow and transport parameters, which cannot be measured directly in most cases A volcanic soils occupy only about 1% of (Jacques et al., 2002). Also, when considering layered soil the terrestrial surface (FAO, ISRIC, ISSS, 1998), profiles, the number of transport parameters may increase they are very important because they are among the most considerably. productive soils of the planet. However, little research Miscible displacement experiments are suitable for calhas been done on their transport properties (Magesan ibrating solute leaching models since they provide inforet al., 2003). In the Canary Islands (Spain), volcanic soils mation about such processes as preferential flow, hydroare crucial since they produce 90% of the main export dynamic dispersion, ion exchange, and adsorption under crops, bananas (Musa acuminata Colla) and tomatoes various flow rate and soil water content conditions (Er(Lycopersicon esculentum Mill.). These soils exhibit spesahin et al., 2002). These experiments usually involve the cial properties as a result of the strong natural aggregaapplication of a solute pulse at the soil surface, followed tion of particles, the high concentration of Fe and Al by measurements of the solute flux and/or resident conoxihydroxides, and the presence of allophanic clays with centration in the profile. Obtaining solute breakthrough large specific surface area and water affinity (Moldrup curves (BTCs) from large soil monoliths is not an easy et al., 2003; Regalado et al., 2003). Considering the strong task. Traditional techniques for solute concentration meaand stable natural aggregation of volcanic soils, the soil surements (e.g., soil coring and solution extractors) are liquid phase is often assumed to be divided into two water usually inappropriate for obtaining high quality data regions: a mobile (dynamic) phase and an immobile (stagwith good spatiotemporal resolution. For this reason, nant) phase associated with the less permeable region of time domain reflectometry (TDR) has become increasthe soil matrix (Mallants et al., 1994). Evidence of the ingly popular as it allows for continuous and simultanepresence of mobile and immobile regions in the soil of ous measurements of the soil water content (") and the electrical conductivity (EC) of the soil solution. When A. Ritter and C.M. Regalado, Instituto Canario de Investigaciones the tracer is a saline solute, and for certain temperature Agrarias (ICIA), Apdo. 60, 38200, La Laguna, Spain; R. Muñozconditions and low background salinities, changes in EC Carpena, Agricultural and Biological Engineering Dep., University can be linearly related to changes in the solute concenof Florida, 101 Frazier Rogers Hall, P.O. Box 110570, Gainesville, FL 32611-0570; M. Javaux and M. Vanclooster, Dep. of Environmental tration. Time domain reflectometry is a less-destructive Sciences and Land Use Planning, Unité Génie Rural, Université Cathand more cost-effective method enabling continuous readolique de Louvain, Croix du Sud, 2, BP2, B-1348 Louvain-la-Neuve, ings at different soil depths (Vanclooster et al., 1995). Belgium. Received 21 June 2004. Special Section: ZNS’03 Vadose Zone Research. *Corresponding author ([email protected]). Abbreviations: BTC, breakthrough curves; CDE, convective–dispersive equation; EC, electrical conductivity; GMCS, global optimization Published in Vadose Zone Journal 4:300–309 (2005). doi:10.2136/vzj2004.0094 algorithm; MIM, mobile–immobile model; NMS, Nelder–Mead–Simplex; nMSE, normalized mean squared error; TDR, time domain re Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA flectometry. 300 Published online May 13, 2005