4.3 Effects of mineral precipitation within the soil matrix
Deposition of minerals under 1-D saturated flow transport conditions was modelled using both B-dot and PHRQPITZ activity coefficient models. However, the results presented here belong only to the PHRQPITZ runs, as this model was concluded to be a better predictor of deposition and solubility behavior for saline water from prior sections of the study.Figure 6 presents the mineral deposition profile with depth from seawater for the three different soil types at the one-year mark of irrigation. The minerals dominating deposition were carbonates and sulfates, namely, gypsum and dolomite for AD134; dolomite and magnesite for AD146; and, calcite, gypsum, and dolomite for AD157. Calculations of mineral deposition in this continuous model are based on the degree of supersaturation and might not reflect true deposition for the case of especially dolomite, which requires microbial mediation or the presence of organic polymers for deposition under ambient temperature and pressure conditions (Warthmann, Van Lith, Vasconcelos, McKenzie, & Karpoff, 2000). Hence, the deposition of dolomite in particular can be taken as a surrogate for the precipitation of carbonates of the calcium and magnesium variety.
The zone of mineral deposition in each soil type also presented interesting results. For example in AD134, gypsum deposition peaked from 40 to 70 cm below ground surface (bgs), while dolomite did not peak until a depth of 120 cm bgs and greater. Similarly, dolomite was the dominant mineral deposited in the top half of the AD146 soil column while magnesite was the dominant mineral in the second half of the modelled soil column. AD146 soil column never exceeded equilibrium saturation of gypsum because of this soil type’s low solid-phase gypsum mineral content. In contrast, AD157 displayed dual depositional peaks of calcite (10-20 am and 40-60 cm bgs) with a peak of gypsum deposition in between (20-40 cm bgs) the calcite peaks.
Porosity loss owing to mineral deposition was significant for all soil types (AD134: 38% - 13%; AD146: 37% - 14%; AD157: 33% - 9%), with the mineral deposition peaks correlating well with zones of maximum porosity loss (Figure 6 ) for the modelled soil types AD134 and AD146. In AD134, minimum porosity was observed where gypsum and dolomite deposition peaks occurred, while the same was true for AD146 where the dolomite deposition peak occurred. Conversely, AD157 displayed the greatest porosity loss in shallow layers up to a depth of 45 cm bgs, corresponding to only the first two mineral peaks, i.e. calcite followed by gypsum. From these results of seawater application to soils, it appears that evaporation in shallow calcite-rich soils (AD157) leads to the most significant porosity problems, followed by the deposition of gypsum when seawater flows from gypsum-rich layers to zones with lower gypsum content. Deeper precipitation might be a more challenging operational and land degradation problem because the soil at depth cannot be easily excavated or rehabilitated between halophyte planting cycles. Furthermore, potential porosity loss problems might require a different mode of irrigation, such as blending of seawater with freshwater, or alternating between periods of seawater and freshwater (or brackish water) application.