SPATIAL DISTRIBUTION OF BORON
tabase accessible to the public (TWDB 2018c). Maps were created with ArcGIS (Esri, Redlands, California). The wells are used for domestic (108 wells), pub- lic (37 wells), stock (31 wells), irrigation (14 wells), and industrial (4 wells) pur- poses (Figure 1). Of the four remaining wells, two had unknown uses and two were not used, except for monitoring groundwater. Wells in the four largest- use categories mentioned above are distributed throughout the study area (Figure 1). Most recent boron concentra- tions within the 2015 to 2018 time in- terval were compiled in this study. Nor- dstrom (2003) described procedures for collecting and chemically analyzing the water samples. Data were non-normally distributed; therefore, non-parametric statistics were used to measure associa- tions between variables, and to evaluate differences in concentration among cat- egories of wells.
Results and Discussion Well depths in the dataset range
from 20 m to 452 m, with a median depth of 98 m (Table 1). Both relatively shallow and moderately deep wells are located in northern and central parts of the study area (Figure 2). Predomi- nantly deeper wells occupy the south- ern part of the study area (Figure 2). Most of the wells in the northern part of the study area are screened in sand and sandstone (Figure 3). The remain- ing wells tap predominantly limestone.
Table 1. Summary of Well Depths and Boron Concentrations N
Well Depth (m) Boron (ug/L)
145 198
Minimum 20
<50
Boron concentrations showed considerable variability, ranging from less than 50 ug/L to 3,390 ug/L, with a median concentration of 102 ug/L (Table 1 and Figure 4). The median boron concentration was closer to the minimum concentration than the maxi- mum concentration, a characteristic of non-normally distributed water quality data. Only two boron measurements exceeded the 2,000 ug/L drinking water advisory level for children, and none exceeded the 5,000 ug/L advisory level for adults. However, 11 observa- tions exceeded the 1,250 advisory ug/L level for sensitive crops. Cotton, the main irrigated crop in the study area, is semi-tolerant of boron in irrigation water (USSL 1954). The highest boron concentrations in groundwater gener- ally occupied the northern part of the study area, where cotton is grown (Figure 4). Boron concentrations were inversely associated with well depth, with a Spearman correlation coefficient of -0.21 and probability level of 0.01. Thus boron concentrations tended to be
Median 98
102
Maximum 452
3,390
higher in shallower wells. A tendency for higher boron concentrations in shal- lower wells may indicate boron sources originating at or near the land surface, such as salt buildup in the vadose zone. However, aquifer composition also varies with well depth and likely influenced the boron concentrations in groundwater. Wells open to sand and sandstone of the Trinity Group had a median depth of 76 m, much lower than the median depth of 111 m for wells open to limestone in the Edwards Group. Boron concentrations were much
higher in wells open to the Trinity Group (median 527 ug/L) compared to those tapping the Edwards Group (median 80 ug/L). These differences were statistically significant, with a Kruskal-Wallis probability level less than 0.0001. A tendency for higher boron concentrations in the Trinity Group is consistent with variations in rock composition in the aquifer. Based on several case studies, Hem (1992) re- ported an average boron concentration of 90 ppm for sandstone and 16 ppm for
Figure 2. Well depth categories (circles); from smallest to largest, circles designate 20-100 m, 101-200 m, 201-300 m, and 301-451 m; north toward top.
58 TPG •
Apr.May.Jun 2019
www.aipg.org
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
