Geologic framework of aquifer units and ground-water flowpaths, Verde River headwaters, north-central Arizona

Abstract

In this chapter, graphical methods to plot geochemical and isotopic data are used to characterize major aquifers and springs discharging to the upper Verde River and to identify changes in water chemistry along the main groundwater flowpath from Big Chino Valley to upper Verde River springs. Samples were analyzed for major and trace elements, δ 18 O, δD, 3 H, 14 C, and 13 C. Groundwater samples are grouped by aquifer, altitude, and geographic location to identify important processes and trends. Sample groups include (a) high-altitude areas west and south of Big Chino Valley, (b) the carbonate aquifer north of Big Chino Valley and the upper Verde River (Mississippian-Devonian, or M-D sequence), (c) the Little Chino basin-fill aquifer, (d) the Big Chino basin-fill aquifer, (e) the carbonate aquifer near the outlet of the Big Chino basin-fill aquifer (Devonian-Cambrian, or D-C zone), and (f) low-altitude springs discharging to the upper Verde River. Limitations of the stable-isotope data used in this study include not being able to volumetrically weight contributions from different areas of the groundwater system. Also, evaporation has had a significant effect on the stable-isotope composition of spring-fed lakes and some samples from the M-D carbonate aquifer. Despite these drawbacks, the Little Chino basin-fill aquifer and the M-D sequence still could be largely excluded as major sources to upper Verde River springs. Low-altitude springs discharging from the Little Chino basin-fill aquifer are ~0.4 per mil (‰) enriched in δ 18 O, with high dissolved strontium (> 450 µg/L Sr) resulting from contact lati-andesite in northern Little Chino Valley. In contrast, the M-D sequence is depleted by about 1.3‰ δ 18 O and 7.9‰ δD and is low in strontium (less than 120 µg/L Sr), compared to moderate values for upper Verde River springs (346 to 440 µg/L Sr). Water chemistry of upper Verde River springs has characteristics of both the Big Chino basin-fill aquifer and the D-C zone of the carbonate aquifer near Paulden. Values of-10.3±0.2‰ δ 18 O and-74.4±2.0‰ δD were used to trace a flowpath from the Big Chino basin-fill aquifer near Paulden through the D-C zone to upper Verde River springs. Disproportionate increases of the boron and lithium along the flowpath of 274 percent B versus 188 percent Li (with no corresponding change in δ 18 O, δD, Ca, and Sr values) indicate the major process responsible for increases in trace elements is water-rock interaction. The upper Verde River springs samples have moderately high values of 17-29 µg/L As, 136-270 µg/L B, and 28-49 µg/L Li, attributed to water-rock contact with marine shale within the D-C zone. The highest concentrations in the study area of 33-38 µg/L As, 330-460 µg/L B, and 54-86 µg/L Li are found in the D-C zone. The presence of measurable tritium and elevated 14 C activity near the outlets of the basins indicates that recharge is occurring beneath major drainages, including Williamson Valley Wash, lower Big Chino Wash, Granite Creek, and Little Chino Creek. Carbon-14 activiti...

Distinguishing Sources of Ground Water Recharge by Using δ2H and δ18O

“…Infiltration and recharge of precipitation through the mountain block in the Little Chino subbasin are occurring but not at a rate significantly higher than through channels at lower elevations (Figure 5). The Bradshaw Mountains are composed of Precambrian granitoid surfaces that are fairly impermeable (DeWitt et al 2005); however, a δ 18 O value of −11.13‰ was measured in a ground water sample issuing from a spring at an elevation of 2014 m (Figure 8). Thus, a portion of precipitation is percolating through the mountain block and potentially contributing to the regional aquifer.…”