3.4.1. Aggregate Water Quality
We examined aggregate water quality parameters between Ogallala and Dockum groundwater wells from recent quarterly sampling in 2014–2015. First, we examined the differences in water quality at a higher level using general statistics for all wells in the study area as provided in
Table 4. Second, we looked at the differences in the aquifers for similar locations through a series of plots in
Figure 8, meaning those wells that are close enough in proximity that many land owners could consider using either water source individually or blend the two according to their needs in total water flow or water quality.
Considering the entire group of samples, the SPC is higher on average in the Dockum Aquifer by a factor of two and range of values is also much higher (500 mg/L range in Ogallala compared to 1600 mg/L range in Dockum). In total alkalinity (TA), the mean between the aquifers is not terribly different, but the range again is much greater in Dockum groundwater. Alkalinity is higher when the total amount of carbonate species is higher, and the presence of more carbonate species generally increases pH. At the same time, the presence of more TA indicates that buffering capacity is higher. Increased buffering and increased pH often seem to be the case in Dockum waters as compared to Ogallala. The upper range of pH is higher in Dockum water (9.4 vs. 8.4) and the mean pH is also higher in Dockum samples. Concerning variability of total alkalinity, Dockum waters certainly have some instances of very high alkalinity (>300 mg/L as CaCO3, 56% of samples) and thus high buffering. At the same time, a smaller fraction of Dockum samples (6%) have total alkalinity fairly low (<125 mg/L as CaCO3). For these low alkalinity samples, their buffering capacity is even lower than the lowest values for Ogallala groundwater (minimum 156 mg/L as CaCO3).
The SAR and the total hardness (TH) in these samples are related according to their definitions. SAR is a ratio between multivalent cations of Mg2+ and Ca2+ relative to monovalent Na+. Total hardness is primarily a measure of the charge equivalent concentration of Mg2+ and Ca2+. Dockum water is more variable in TH with a mean value that is half of Ogallala groundwater. At the same time, the mean SAR for Dockum groundwater is very high at a value of 19 while Ogallala water has a mean value almost 20 times less (1.3). In general, sodicity effects must be managed at value of SAR > 9. All of the Ogallala groundwater we sampled was <9 SAR. The range and the overall values for Dockum groundwater were much higher with 65% of Dockum samples at SAR > 9. The trends of lower total hardness in the Dockum combined with higher SAR are related. The relative and the total amount of Na+ increases in many Dockum samples while at the same time Mg2+ and Ca2+ decrease. Thus, the Dockum has softer water compared to the Ogallala, but this coincides with a general increase in sodicity hazard.
We next chose to examine spatial differences in Ogallala and Dockum waters using spatial groupings as provided in
Figure 1. Each spatial grouping has at least one well sampling location that is a Dockum well and one that is Ogallala. (Note these groupings are different from the historical analysis groupings which covered a much larger area.) All groupings have at least two wells for each aquifer in the group except for the west group, which only has one well for each aquifer. These groups are used for Dockum and Ogallala box plot comparisons on different aggregate water quality parameters as given in
Figure 8. The logic behind the comparison is that a single property owner might have access to both aquifer waters. They would want to know salient differences in their two water resources.
Examining each water quality parameter in turn, we find differences between Ogallala and Dockum waters within the same group, differences in the same aquifer but in different spatial groups, and spatial trends in the way in which common location Ogallala and Dockum waters differ from each other. For SPC overall values, the trend is that SPC is higher in Dockum waters as compared to Ogallala waters within each spatial group. The higher TDS is in agreement with the overall trend in higher salinity for Dockum waters. However, the degree of the difference is certainly different. The ratio of mean values of Dockum TDS to Ogallala TDS within the same spatial group ranges from 1.3 to 3.4 with the highest value being in the north and the lowest in the west. The north, while it has the highest SPC multiple relative to Ogallala water, it is not the highest absolute difference. The greatest absolute increase in SPC is in the southwest region, where SPC is on average 1150 μS/cm (750 mg/L TDS equivalent) higher in Dockum groundwater as compared to Ogallala.
Considering SPC variability, the plots show distinctly that there is much more variability in Dockum waters over Ogallala waters within each spatial group as well. This is certainly true in the north, central east, and central west groups where interquartile ranges (IQRs) in Dockum are 540–750 μS/cm SPC compared with IQRs of 38–200 μS/cm SPC in Ogallala waters.
SPC variability for Dockum water arises because of both differences in time at the same well location and differences between locations in the same aquifer.
Table 5 presents standard deviations in differing Dockum spatial groups when time is held as constant, when location is held as constant, and when both are allowed to vary for SPC (μS/cm). The central east group, which has a high amount of Dockum samples (n = 14), has the same location, a temporal standard deviation of 160 μS/cm (four sampling events) with the same season, across a location standard deviation of 450 μS/cm. Within this spatial group then, the high IQR for SPC is due in a larger measure to variation in location rather than variation within the year. The same greater interspatial variation is seen in the central west and the study area as a whole. Contrasted with this is north group where temporal standard deviation is 390 μS/cm and spatial standard deviation is 380 μS/cm. For north and southwest groups, variation is relatively even in both space and time. Thus, the source of large salinity variation as measured by SPC in Dockum groundwater is sometimes due in greater measure to spatial variation over temporal variation, and in other instances there is an equal contribution of variation from both changes in time and changes in space. The inter-annual variation in water quality at a single Dockum location could make blending applications and other suitability evaluations more difficult because that variation could alter the values under which the water is intended to be used.
Looking at aquifer differences in TA, the north through central west groups show little difference between Ogallala and Dockum aquifers. Mean Dockum to Ogallala ratios (TADock/TAOgalla) range from 0.9 to 1.3 while they are 1.4–1.7 in the southwest and west. All locations are well buffered in general at total alkalinity >200 mg/L as CaCO3. Looking at all of the samples seasonally over the one-year time frame, pH changes are in most cases only ±0.2 pH units for most events with just a few changing as much as 0.5. In instances where there are larger TA differences, Dockum waters are higher. These Dockum samples also have higher pH from higher total carbonates compared to same location Ogallala waters.
Figure 8 reveals that total hardness is one of the biggest areas of difference in the two aquifers within each spatial group. With the exception of the north spatial group, the hardness seems much lower in the Dockum compared to Ogallala water with Dockum-to-Ogallala ratios of 0.04–0.30 (Ogallala 3–25× harder than Dockum). Contrastingly, in the north group, the ratio is 1.72 (Dockum water harder than Ogallala). Looking at the absolute level of hardness, in general many Dockum waters are relatively soft, having total hardness < 90 mg/L as CaCO
3 (31/50 = 62%). For the southern Great Plains region, 90 mg/L as CaCO
3 is relatively soft. Compared to Ogallala waters in the same areas, TH > 200 mg/L CaCO
3 in 87% of all samples. In the four softer water regions (central east–west), Dockum waters have a cationic composition that is low in multivalent Ca
2+ and Mg
2+ while at the same time having higher overall TDS. The additional cations corresponding to high SPC come from higher quantities of K
+ and Na
+.
As mentioned, the north group is very different comparative hardness between Ogallala and Dockum waters. This entire area shows that the locations that are called Dockum are in a region where the distinction between Dockum and Ogallala units is less easy to discern. Indeed, in places where there truly is a Dockum outcrop, there is really only one water-bearing unit at all. The well depths in the north are also much shallower than other Dockum locations we sampled, and the locations are very close to the Canadian river. Northern differences from other Dockum locations and higher variability are likely a result of closer surface hydrologic interactions.
Generally, SAR is higher in all of the spatial groups for Dockum waters over Ogallala with the increase ranging from 10 to 24× higher in the Dockum. The large increase might not be as much of an issue if there were not so many SAR values > 9. However, out of all Dockum samples 66% were at SAR 9 or greater. A few samples in the central east were lower than this, and all of the north group samples had SAR < 9. For Ogallala samples, 89% had SAR < 2, and the maximum value was only 5.1. The southwest group stands out as having very high SAR at 56 ± 7.1 (mean ± 95% conf). There is no possibility of this water being safely used for any type of irrigation without substantial treatment or modification. The risk to soil structure and soil health is too great.
3.4.2. Detailed Hydrochemistry
We now examine Dockum and Ogallala waters at the more detailed level—the results of individual water constituents. We first used Piper diagrams to classify aquifer waters in hydrochemical facies (
Figure 9,
Table 6). We then employed principal component analysis (PCA) to examine common constituent groups which vary together to better discern how and why water quality is changing in space, time, and between aquifers (
Figure 10,
Figure 11 and
Figure 12).
Concerning alkali type (classes 1 or 2), Dockum samples were nearly all alkaline-dominated (76%, class 1 in Piper diagram) by an average Na+K total cation fraction of 77 ± 25% (mean ± sd) while Ogallala waters were predominantly majority alkaline earth (91%, class 2 in Piper) at a Mg2++Ca2+ cation fraction of 73 ± 11%. There were more exceptions to the overall trend in the Dockum samples compared to Ogallala. In the north area, Dockum wells (four of them, tan filled diamonds) exhibited a mixture of trends. Two locations were consistently alkaline predominant (61–90% Na+K). One location was always right around 50% of each cation type. Another location was consistently alkaline earth dominant (21–23% Na+K). There was also one Dockum location that was evenly balanced between alkaline and alkaline earths in the central east group.
Looking at cations more specifically, the reason that Ogallala samples are more often majority alkaline earth over alkaline is that the cationic description for nearly all samples is either >50% Mg
2+ or mixed cation (classes 6 and 9 in
Figure 9). In Dockum samples most of the waters (76%) are solidly in cation class 7 (Na+K > 50% of cations) with remainder in mixed class 9. In general, the further south and west in the study area that one moves, the higher the fraction of cations which are Na+ for Dockum. The southwest region had the highest concentrations of dissolved Na
+ at 420–520 mg/L.
In acid types (strong class 3 or weak class 4), all Ogallala samples were weak acid-dominant while Dockum samples were more evenly split between strong and weak acid (39%:61% strong-to-weak). Spatially, Dockum water samples were almost entirely weak acid-dominant in southwest and west groups (91% for both together) while the other three spatial groups were more evenly mixed (51% weak acid). Specifically, the north group was more strong acid (67% of samples) while central west was split exactly evenly, and central east was more weak acid (75% of samples). Combining these trends with aggregate parameters, the southwest and west groups (weak acid predominant) were also both low in total hardness for all samples (<45 mg/L as CaCO3 TH) and generally had high SAR and total alkalinity as well. These three characteristics are not entirely unique to only west and southwest groups (central west was also high in SAR and low in Total Hardness). It is fair to say that the data do show that the proportion of weak acids over strong does increase moving from east to west and north to south in the Dockum aquifer within the study area. The north group does not always follow that trend because water quality there is so variable generally (in the outcrop), but the spatial trend does lean as described.
A more detailed look into anionic composition for the waters shows that, as expected, Ogallala water is dominated by HCO
3- (weak acid). In the Dockum, the largest anion class is like the Ogallala, class 6 HCO
3-, but a sizeable amount is still in classes 9–11 as mixed anion, Cl
-, and SO
42- dominant. The Dockum water table samples (filled in tan diamonds in
Figure 9) have the most variation of out of all samples and have individual sample results which are in all four anion classes (6–9).
The facies mapping in the central diamond shows nearly all Ogallala samples decidedly in class 13 Mg-HCO3 with a smaller proportion of samples in the central mixed region (17) due to elevated quantities of alkali cations. In the Dockum aquifer, it is seen that the unconfined outcrop region samples (all in the north) vary between mixed classes (17–18), one sample which is Mg-HCO3 like Ogallala waters, and a few samples which are class 15 Na-Cl. Nearly all of the confined Dockum samples are either Na-Cl or Na-HCO3 type. To the degree that the unconfined Dockum region represents recharge generally, it makes sense that these samples are more varied in water quality (due to seasonality, rainfall, and potential exchange with the nearby Canadian River) and that they are more dominant in alkali earth divalent cations, likely derived from surface dolomitic minerals. When Dockum waters have descended beyond the recharge zone and into the confining unique, they have experienced a marked increase in Na+, having lost nearly all of their alkali earths, presumably through ion exchange in soils. Any SO4 and Cl ions, though they may not have been reduced in absolute terms, have been dwarfed in relative terms by HCO3- arising from mineral dissolution.
Figure 10 shows in two panels how the constituents in the PCA work together to reduce the overall number of dimensions from eight to three by visualizing the constituent loadings per factor. Factor 1 (F1) has high positive loadings from
,
, and
and moderate negative loadings from
and
. Factor 2 (F2) has high positive loadings from
,
, and
and a moderate positive loading from
. Factor 3 (F3) has high positive loading from fluoride and a moderate positive loading from
. It also has a slight negative loading from
. A general interpretation of these factors, by their constituents is as follows. F1 represents the influence of multivalent cations and nitrate, while at the same time deemphasizing sodium and boron. F2 represents the collective influence of sulfate, sodium, boron, and to a lesser degree bicarbonate. Additionally, F3 is strongly indicative of fluoride’s influence with some co-influence of bicarbonate.
Looking at the two figure panels, it is also possible to see how the constituents cluster together generally. Boron and sodium are highly correlated with each other in all three factors and in the dataset generally (Pearson R = 0.85, p < 0.01). Bicarbonate is closely connected to their cluster with the important distinction that moderate loading on F3 while B-Na do not. Sulfate is also closely correlated to B-Na with the difference in it being that it loads much more strongly on F2, and it has no negative loading on F1. Additionally, highly clustered is magnesium, calcium and nitrate. Magnesium and calcium are more closely correlated to each other than to nitrate. Fluoride is the most different from all of the other variables. It loads weakly on every factor except for F3 where it correlates some with bicarbonate.
PCA score plots with box plot distributions by factors are presented in
Figure 11. The use of the score plots (a, b) gives some insight into aquifer types and clustering that occurs from unsupervised PCA. Ogallala wells cluster in the center of the three-factor space. Most of their variability is captured in F1 indicating that their water quality variation exists primarily in
,
, and
. The Ogallala cluster exhibits only minor variation in F2 and slightly more variation in F3. Thus, the quantities of
,
,
,
and
do not vary greatly amongst all of the Ogallala wells. This is seen more clearly in
Figure 11d,e where the interquartile range for each of these five constituents in the Ogallala wells is fairly small. For example, in panel (d) it can be seen that the IQR for
and
varies much less compared to both Dockum water types. The order of magnitude in these IQR is 0.16 and 0.30, respectively, compared with values of 0.35–0.52 and 0.61–0.85, respectively, in Dockum waters.
The Dockum confined wells cluster strongly due to strong invariance in F1, the factor for multivalent cations and nitrate. Looking at
Figure 11c, the overall concentration of these
and
ions is generally lower with values mostly at or below 10 mg/L. Their variation, however, is still substantial with an IQR of 0.51 and 0.75 log units, respectively.
is lower in this class of water compared to the other two, but its IQR is also high on the scale at 1.00 log units.
is the highest in Dockum confined waters, and, because sodium is negatively loaded onto F1, this also contributes to these waters bearing consistently negative scores.
What differentiates samples in the Dockum confined waters is F3 scoring and F2 to a lesser extent. Though F2 explains more of the overall variation in the dataset (30% compared to 20% in F3), F3 elucidates more differences in Dockum confined waters.
Figure 12 illustrates the differentiation in both Dockum confined and water table groundwater. In the figure, we have divided the space of fluoride and bicarbonate results into four quadrants with thresholds of above or below 2 mg/L F
- and 300 mg/L HCO
3-.
For the confined Dockum groundwater, 71% of all of the samples are above both thresholds which would yield a high F3 score as can be seen in
Figure 11b where a sizeable majority of confined Dockum samples are positively scored on F3. Additionally, what can be seen in
Figure 12 is that, even though most of the confined Dockum groundwater results are in the upper right quadrant, there is a wide spread of concentration values for both fluoride and bicarbonate, which corresponds to high variation in F3.
To summarize the PCA, variation in Ogallala groundwater quality in this region is determined in large part by what might be conceived of as a common hydrogeochemical source or process which is highly correlated to Ca, Mg, and NO
3. Despite all of the water quality constituents, these three analytes differentiate Ogallala water quality profiles more than any other. Since NO
3 is sourced from infiltrated source water, it is logical to suppose that the significance of this common factor is that it indicates the strength of the connection to surface infiltration. For the Dockum, the unconfined water quality is differentiated mostly by the same ions as the Ogallala with the addition of
and
. Between confined and unconfined Dockum sections, the differentiation comes from
and
. Fluoride in particular is known to be high in the Ogallala and is supposed to be sourced from volcanic ashes mixed in with Ogallala sediment [
49,
50]. It does not to appear to be known if
would be found in native Dockum sediment, and thus it seems possible that higher
in the confined Dockum sediment indicates cross-flow between the Ogallala and Dockum and that something about the presence of
is connected to a correlation with
.