Diel and Seasonal Patterns in Continuously Monitored Water Quality at Fixed Sites in Two Adjacent Embayments of the Tidal Freshwater Potomac River

Alexander Graziano,R Jones

doi:10.3390/w9080624

Abstract

Water quality patterns in two adjacent embayments of the tidal freshwater Potomac River were compared using continuously monitored data. The two embayments, the tidal Occoquan and Gunston Cove, are shallow embayments fed by tributaries which vary in their watershed attributes. The tidal Occoquan has a larger watershed and a dam just upstream from the head of tide whereas Gunston Cove has a large wastewater treatment plant just upstream of the embayment. Water temperature, specific conductance, dissolved oxygen (DO), and pH were collected at 15 min intervals at representative locations in the two embayments from April through November of 2012. Mean daily values of water quality variables exhibited similar seasonal patterns at both sites which were related to seasonal patterns of forcing functions such as temperature, light and freshwater inflows. Three two-week intervals were examined in great detail. During a mid-summer period of stable conditions and low flow, both embayments exhibited strong and consistent diel (light cycle-based) periodicity in temperature, DO, and pH. Specific conductance exhibited semi-diel (tidal cycle-based) periodicity. The two other two-week intervals were punctuated by substantial flow events which resulted in major changes in daily mean values and disruptions to the typical diel or semi-diel cycles. The large rainfall event related to Hurricane Sandy caused a major disruption in diel and semi-diel cycles. Some cycles were quickly re-established and others took a week or more. The two embayments exhibited similar behavior in most aspects. Site-specific factors such as differences in the frequency and magnitude of tributary flow pulses and the presence of treated sewage discharge at Gunston Cove helped explain some of the differences observed between the two embayments.

Highlights

Water quality in tidal systems is highly dynamic and can exhibit predictable daily rhythms or undergo rapid directional changes. Capturing and characterizing these properties is the goal of continuous monitoring which can be defined as the acquisition of water quality data at short intervals over long periods of time (e.g., 1 year) [1]
Mean daily dissolved oxygen (DO) and pH were negatively correlated with tributary flow at each site, indicating that flushing might be a controlling factor when inflow was elevated
These relationships were further explored by closely observing the behavior of 15 min values over selected approximately two-week periods

Summary

Introduction

Water quality in tidal systems is highly dynamic and can exhibit predictable daily rhythms or undergo rapid directional changes. Capturing and characterizing these properties is the goal of continuous monitoring which can be defined as the acquisition of water quality data at short intervals (e.g., every 15 min) over long periods of time (e.g., 1 year) [1]. Over the last two decades, continuous monitors have been deployed across estuarine systems worldwide. In the Chesapeake Bay region, for example, large scale monitoring efforts, such as the Virginia Estuarine and Coastal Observation System (VECOS) (http://web2.vims.edu/vecos/) and Maryland’s “Eyes on the Bay” program (http://eyesonthebay.dnr.maryland.gov/) have been used. Water 2017, 9, 624 to track changes in water quality and assess the overall health of the bay. Only a relatively small amount of data from these and other monitors has been rigorously analyzed and published

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