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.