Regional oxygen reduction and denitrification rates in groundwater from multi-model residence time distributions, San Joaquin Valley, USA

Abstract

Rates of oxygen and nitrate reduction are key factors in determining the chemical evolution of groundwater. Little is known about how these rates vary and covary in regional groundwater settings, as few studies have focused on regional datasets with multiple tracers and methods of analysis that account for effects of mixed residence times on apparent reaction rates. This study provides insight into the characteristics of residence times and rates of O 2 reduction and denitrification (NO 3 − reduction) by comparing reaction rates using multi-model analytical residence time distributions (RTDs) applied to a data set of atmospheric tracers of groundwater age and geochemical data from 141 well samples in the Central Eastern San Joaquin Valley, CA. The RTD approach accounts for mixtures of residence times in a single sample to provide estimates of in-situ rates. Tracers included SF 6 , CFCs, 3 H, He from 3 H (tritiogenic He), 14 C, and terrigenic He. Parameter estimation and multi-model averaging were used to establish RTDs with lower error variances than those produced by individual RTD models. The set of multimodel RTDs was used in combination with NO 3 − and dissolved gas data to estimate zero order and first order rates of O 2 reduction and denitrification. Results indicated that O 2 reduction and denitrification rates followed approximately log-normal distributions. Rates of O 2 and NO 3 − reduction were correlated and, on an electron milliequivalent basis, denitrification rates tended to exceed O 2 reduction rates. Estimated historical NO 3 − trends were similar to historical measurements. Results show that the multi-model approach can improve estimation of age distributions, and that relatively easily measured O 2 rates can provide information about trends in denitrification rates, which are more difficult to estimate.

A Bayesian approach to infer nitrogen loading rates from crop and land-use types surrounding private wells in the Central Valley, California

“…Landon et al (2011) did not find significant decreases in nitrate concentration in wells due to denitrification. Results of a multimodel averaging approach to estimate oxygen and nitrogen reduction rates in the San Joaquin Valley did estimate denitrification rates to be significant (Green et al, 2016). However, Green et al (2016) also estimated oxygen reduction rates to be low with a median of 0.12 mg L −1 yr −1 .…”

“…Results of a multimodel averaging approach to estimate oxygen and nitrogen reduction rates in the San Joaquin Valley did estimate denitrification rates to be significant (Green et al, 2016). However, Green et al (2016) also estimated oxygen reduction rates to be low with a median of 0.12 mg L −1 yr −1 . Much of the shallow groundwater in the CV is well oxygenated: the dissolved oxygen content of Lockhart et al (2013) study wells with a measurement (Table 1) was above 5 mg L −1 on average .…”

“…Much of the shallow groundwater in the CV is well oxygenated: the dissolved oxygen content of Lockhart et al (2013) study wells with a measurement (Table 1) was above 5 mg L −1 on average . In addition, Green et al (2016) found that the estimated rates of oxygen and nitrogen reduction would not protect wells from nitrate contamination, given current nitrogen application rates. We therefore do not expect that denitrification or ammonium volatilization had a significant, overall effect on our model results, but rather may have had an isolated effect in areas with Rice and possibly with Manured Forage.…”

Comparison of groundwater age models for assessing nitrate loading, transport pathways, and management options in a complex aquifer system

“…Generally, groundwater with younger ages tends to show rapid response to fertilizer applications and higher vulnerability to the accidental release of contaminants at the surface (Manning et al, ; Visser et al, ). By utilizing the estimated groundwater age and the nitrate loading history, the response of the aquifer to the historical nitrate loading can be evaluated (Green et al, , ; Katz, Böhlke, & Hornsby, ; Koh et al, ; McMahon et al, ).…”

“…Although the LPM can provide appropriate estimation of the groundwater age distribution for properly conceptualized aquifer systems, its applicability may be limited in settings with multiple and discrete groundwater flow components, such as with the perched layer in this study. Under a complex hydrogeologic setting, therefore, the use of multimodal LPMs (e.g., Green et al, ; Jurgens et al, ) on the basis of the proper conceptual model and adequate selection of the age tracers could be recommended to better characterize age mixing phenomena (Green et al, , ; Leray, Engdahl, Massoudieh, Bresciani, & McCallum, ; Marçais, Dreuzy, Ginn, Rousseau‐Gueutin, & Leray, ). Our study also demonstrates that despite potential uncertainties caused by various input parameters (i.e., aquifer geometry, hydraulic conductivity, porosity, and recharge rate), the numerical models combined and calibrated with the tracer concentration could also be an alternative method to characterize the multiple recharge pathways and multimodal age distributions.…”