3.2. Relationship between Soil Salt Content and Chemical Components of Groundwater
The analytical results of several soil profiles were used to evaluate the relationship between soil salt accumulation in different soil layers and underground water quality [45
]. Section points P1’–P2’–P3’–P4’–P5’–P6’ from northeast to southwest collected in June were used as typical section points on the basis of runoff direction and soil salinization degree, and irrigation water samples S1’–S2’–S3’–S4’–S5’–S6’ from nearby wells of the soil samples were used as typical water samples, as shown in Figure 9
The soil salt ions are shown in Figure 10
. The main cations are Ca2+
, and the main anions are SO42−
. Therefore, the main components of soil salinization are SO42−
, which are influenced by topography, geomorphology, hydrology, meteorological conditions, and human activities.
A comparison of different soil layers indicated that the salt content and ionic components have large spatial variability rather than a gradually increasing trend. The variation coefficients of total soil salinity SO42− and CO32− in the soil layers with depths of 0–10 cm and 10–60 cm are relatively large, indicating strong spatial variability.
The migration law of salt transport revealed that Cl− is the most active ion, followed by SO42−. CO32− is stable. However, in practice, saline soil in irrigated areas is mainly SO42−, followed by Cl−, because SO42− is abundant in the soil parent material, resulting in a higher content of SO42− radical ions in groundwater than that of Cl− ions. In addition, HCO3− under drought conditions becomes CO2 via intense evaporation, thereby causing CO32− to increase the content of CO32− in topsoil. Salt accumulation and desalination coexist on the soil soluble salt of the irrigation district and result in a higher value of soil salt ions or total salt content of the 0–10 cm soil layer than that of the 10–60 cm soil layer. Salt accumulation is mainly due to salt activity, and desalination is not evident due to the climatic conditions of the irrigation area, that is, low rainfall evaporation intensity has a close relationship with salt accumulation.
The change in groundwater quality in Figure 11
is relatively consistent with the salt and ionic component contents of the 0–10 cm soil layer. The change in the mineralization of groundwater from S1′ to S6′ shows a gradual upward trend, and the chemical components of groundwater are dominated by SO42−
, and Na+
3.3. Correlation Analysis between Soil Salinity and Groundwater Quality
(a) Correlation of surface soil salinity and groundwater
We used surface soil salinity and underground water quality in the study area as the study objects. We utilized total soil salt contents of SO42−, Cl−, Ca2+, Na+ + K+, Mg2+, and HCO3− and pH as the basic data and the irrigation groundwater TDS(01) of SO42−(02), Cl−(03), Ca2+(04), Na+ + K+(05), Mg2+(06), and HCO3−(07) and pH(08) as the system factor sequence. Then, the correlation degree between soil salinity and irrigation groundwater quality was calculated and analyzed.
The absolute and relative correlation degrees of groundwater quality to topsoil salinity were obtained based on the data by conducting a non-dimensional quantitative treatment and using Formulas (4) and (6).
The absolute correlation degrees are expressed as
ε01 = 0.605, ε02 = 0.586, ε03 = 0.636, ε04 = 0.560, ε05 = 0.683, ε06 = 0.548, ε07 = 0.511, and ε08 = 0.512.
The relative correlation degrees are expressed as
r01 = 0.786, r02 = 0.755, r03 = 0.825, r04 = 0.704, r05 = 0.871, r06 = 0.674, r07 = 0.545, and r08 = 0.532.
When θ = 0.5, the gray comprehensive relationship degree is calculated using Formula (7); the results are expressed as
ρ01 = 0.696, ρ02 = 0.671, ρ03 = 0.731, ρ04 = 0.632, ρ05 = 0.777, ρ06 = 0.611, ρ07 = 0.529, and ρ08 = 0.532.
The results are represented by a radar map. A radar map can clearly show the correlation between groundwater factors and salt ions in soil. The more peripheral the value is, the stronger the correlation is. The calculation result in Figure 12
shows that the correlation degree between surface soil salinity and groundwater correlation is high and presents a non-consistency characteristic, thereby showing that the accumulated salt in the surface soil has a direct relationship with underground water quality. Groundwater irrigation directly affects soil salinity characteristics. The value of correlation indicates that each component has a different response to the relationship between soil salt content and underground water quantity.
On the basis of the order of the correlation degree value, the result indicates that the contribution of components in irrigation water quality to soil salt accumulation is Na++K+, Cl−, salinity, SO42−, Ca2+, Mg2+, pH, and HCO3− from large to small. The correlation degrees of Na+ + K+, Cl−, mineralization, SO42−, Ca2+, and Mg2+ are >0.6, indicating that these ions within irrigation water are mostly stored in the topsoil. The correlation degrees of pH and HCO3− are less than 0.6, indicating the low representativeness and low response of surface soil salt accumulation. HCO3− is the alkaline component in underground water. The minimum contribution of irrigation water to soil salt accumulation shows that the soil type is saline sodic rather than alkalized.
(b) Correlation of soil salinity at different depths and groundwater
In the study area, soil salinity is influenced by processes of salt leaching due to flood irrigation and evaporation. The soil salinization status was unaffected by groundwater level change when the level below the buried depth of groundwater. Therefore, the correlation degree between salt and irrigation water in the soil profile was analyzed from the perspective of irrigation water quality, and the correlation degree was sorted to determine the influence of the change in irrigation water quality on soil salt. The calculation results are shown in Figure 13
The results showed that the correlation degree between soil salt and irrigation water quality differed at the different depths. The correlation degree in the 0–40 cm soil layers from large to small was Na+ + K+, Cl−, salinity of water, SO42−, Ca2+, Mg2+, pH, and HCO3− in sequence. When the soil depth ranged from 40 cm to 60 cm, the correlation degree changed dramatically. In this layer, the dominant position of Na+ + K+ and Cl− gradually moved downward, and SO42−, Ca2+, and Mg2+ gradually moved upward from the original inferior position and became the main influencing factors of correlation degree. The correlation degree was stable when the soil depth was greater than 60 cm.