The effects of precipitation, river discharge, land use and coastal circulation on water quality in coastal Maine

Abstract

Faecal pollution in stormwater, wastewater and direct run-off can carry zoonotic pathogens to streams, rivers and the ocean, reduce water quality, and affect both recreational and commercial fishing areas of the coastal ocean. Typically, the closure of beaches and commercial fishing areas is governed by the testing for the presence of faecal bacteria, which requires an 18-24 h period for sample incubation. As water quality can change during this testing period, the need for accurate and timely predictions of coastal water quality has become acute. In this study, we: (i) examine the relationship between water quality, precipitation and river discharge at several locations within the Gulf of Maine, and (ii) use multiple linear regression models based on readily obtainable hydrometeorological measurements to predict water quality events at five coastal locations. Analysis of a 12 year dataset revealed that high river discharge and/or precipitation events can lead to reduced water quality; however, the use of only these two parameters to predict water quality can result in a number of errors. Analysis of a higher frequency, 2 year study using multiple linear regression models revealed that precipitation, salinity, river discharge, winds, seasonality and coastal circulation correlate with variations in water quality. Although there has been extensive development of regression models for freshwater, this is one of the first attempts to create a mechanistic model to predict water quality in coastal marine waters. Model performance is similar to that of efforts in other regions, which have incorporated models into water resource managers' decisions, indicating that the use of a mechanistic model in coastal Maine is feasible.

Keywords: Escherichia coli; coastal water quality; multiple linear regression models.

Figure 1.

Land use characteristics of the Saco River watershed. The numbers indicate locations of TRMM measurements.

Figure 2.

Mean daily precipitation (mm) from 5 January 1998 to 5 January 2013. The numbers indicate locations of TRMM measurements. The black square located in TRMM square 8 indicates the location of Environmental Buoy no. 44007. The two rectangles indicate the regions shown in figures 3, 5 and 7. The grey lines indicate the boundaries of the Saco, Androscoggin and Kennebec River watersheds (from southwest to northeast). Inset shows location of the study regions.

Figure 3.

Location of the water quality stations from study I (small grey circles) and study II (large black circles) in the mouths of the Kennebec and Androscoggin Rivers (a) and the Saco River (b). The black lines in panel (b) indicate the locations of the jetties. The grey arrow in panel (b) shows the direction of movement of the river plume due to flood tides and easterly/northeasterly winds. The black arrow shows the direction of the river plume due to ebb tides and westerly winds.

Figure 4.

(a) Average daily precipitation (mm) over the Saco River watershed obtained from TRMM squares, (b) discharge (m³ s⁻¹) of Saco River measured at USGS gauging station in Cornish, ME, (c) salinity at stations within Saco River and Saco Bay, during study II.

Figure 5.

Mean concentrations of faecal coliforms (a) measured at stations in the mouths of the Kennebec and Androscoggin Rivers. (b) Fraction of RWQEs preceded by a large discharge event in the previous 3 days. (c) Fraction of times that a large discharge event did not result in a RWQE in the next 3 days, i.e. a ‘false alarm’. (d) Fraction of RWQEs preceded by a large precipitation event in the previous 3 days. (e) Fraction of times that a large precipitation event did not result in a RWQE in the next 3 days, i.e. a ‘false alarm’ (e). Grey circles indicate stations whose percentage of RWQEs or percentage of false alarms was not significant at 95%.

Figure 6.

Monthly averages of each station in Kennebec and Androscoggin River watersheds (a) and in Saco River watershed (b). Small filled circles represent monthly averages of individual stations. Large filled circles represent monthly average of all stations.

Figure 7.

Mean concentrations of faecal coliforms (a) measured at stations in the mouth of the Saco River. (b) Fraction of RWQEs preceded by a large discharge event in the previous 3 days. (c) Fraction of times that a large discharge event did not result in a RWQE in the next 3 days, i.e. a ‘false alarm’. (d) Fraction of RWQEs preceded by a large precipitation event in the previous 3 days. (e) Fraction of times that a large precipitation event did not result in a RWQE in the next 3 days, i.e. a ‘false alarm’. Note that large black circles indicate location of stations measured during the years 2010–2012. Grey circles indicate stations whose percentage of RWQEs or percentage of false alarms was not significant at 95%.

Figure 8.

(a–e) Daily concentrations of E. coli from 9 November 2010 to 30 November 2012 at the five stations in study II.

Figure 9.

(a–e) Daily total coliform concentrations from 9 November 2010 to 30 November 2012 at the five stations in study II.