Salt chemistry effects on salinity assessment and soil solution modeling in the Arkansas River basin, Colorado

Abstract

Electrical conductivity is an essential indicator of soil quality. Methods used to measure electrical conductivity (EC) were examined to determine the effects of laboratory analysis and extrapolations to in-situ conditions. Methods were tested using combinations of (1) surrogate irrigation waters (SI) to saturate soils over a range of chemical concentrations, (2) soils with different salinity levels, and (3) soils ground or retaining aggregates. Baseline soil EC levels were measured from soil extracts that were saturated with distilled water (ECe) and showed no significant difference between ground and aggregated treatments for the low salinity soil ECe. When the low salinity soils were saturated with SI waters, the response ECs varied as SI concentrations increased. The sum of the baseline ECe and SI EC were not equal to the measured EC above approximately 3.5 dS m-1, suggesting that gypsum dissolution was becoming limited. Soils with high salinity (ECe >8 dS m-1) lacked structure and aggregates and could not be compared to ground soils. None of the tests with the high salinity ground soils had the sum of the baseline (distilled water) ECe and the SI EC equal to the measured EC of soils saturated with SI. Multiple extractions from the same soil sample were processed to determine salt removal potential from calcareous/gypsiferous soil. The Ca concentrations remained relatively constant over 14 extractions while Na concentrations decreased. The ECe decreased from above 8 dS m-1 in the initial extraction to approximately 4 dS m-1 by the 9th extraction, and remained stable to the 14th extraction. This stable ECe suggests that mineral reservoirs of gypsum and calcite remain in the soils. These mineral reservoirs have implications for salinity removal, which becomes limited to the more soluble salts and minerals (e.g. Na and mirabilite). Examination of the multiple extraction data suggests that improved leaching will not successfully lower the EC level below approximately 4 dS m-1 due to the gypsum and calcite reservoirs in the soil. Combinations of the irrigation water chemistry and precipitation and dissolution chemistry can potentially complicate or negate expected leaching potential. Mineralogical variations associated with salinity influence the calibration of the electromagnetic induction meter because the ions are the primary carriers of the electromagnetic resonance. Soils in the high plains of the lower Arkansas River Basin of Colorado are reservoirs of calcite and gypsum. When ions in solution precipitate, their influence on the electromagnetic resonance is decreased. Current EM-38 (Geonics, Ontario, Canada) calibration equations for the lower Arkansas River Basin rely upon electromagnetic measurements in the vertical position (EMv) and water content measurements to predict saturated paste electrical conductivities (ECe). Calibration equations developed in this research, use either depth averaged or depth weighted salt concentrations and/or predicted pore water salt concentrations from Visual Minteq. For example, the current Downstream sub-region calibration equation relating EM readings to soil ECe has an R2 is 0.54 with an root mean square error (RMSE) of 2.16 dS m-1. The equation from this research, using depth weighted Mg concentrations and SAR with Visual Minteq has an R2 of 0.93 with a RMSE of 1.34 dS m-1, and is effective for both the Upstream and Downstream sub-regions. Validation of these equations suggests that predictability is equivalent between the initial sub-region model and the models for the entire region. The inclusion of the chemistry/mineralogy in the calibration equations serves to resolve some of the unevenness of the EMv-ECe calibration, but at the cost of more complex computing and data requirements. However, the inclusion of the chemical data offers an alternate approach not yet utilized in extrapolating the calibration of the EM-38 from a field to a regional scale. Calcareous soils in the Lower Arkansas River Basin are impacted by salt concentrations in irrigation water and high ground water tables. Leaching many of these impacted soils is not effective in reducing salinity due to capillary rise at locations with high water tables. Modeling of management options for these soils can predict potential changes in the salt accumulation in the soil. Three locations with different levels of salinity chemistry and electrical conductivities were modeled in the Hydras-ID version 3.0 (HID) computer program, which has been coupled with UNSATCHEM. This HID program allows modeling of soil hydraulic properties, root growth, carbon dioxide production and transport, as well as primary ion reactions and flux both in solution and as solids. Surface water quality was varied in the model runs, along with the depth to ground water and cropping options. Soil surface evaporation led to evaporites forming on the soil surface for most model runs, particularly for the fallow runs. The ground water was predicted as a source of salts in the modeled soil profile. The uptake of water by the crop was found to be a driving factor in the location of the salt accumulation within the soil profile. Changes in irrigation water quality influenced the magnitude of the peak salt concentrations but not the depth of those peaks. The volume of water applied altered the depth to the peak salt concentrations; additionally, the ground water also contributed to the depth to the peak salt concentrations. Precipitates of gypsum and calcite were the primary repositories of precipitated salts in the soil profile and were subject to redistribution within the soil profile. It is concluded that no single management option will lead to the salinity decreases needed to improve crop production. Potentially costly management options are required to improve soil salinity problems.