An investigation of spatial and temporal drinking water quality variation in green residential plumbing

Highlights

• One-year intensive drinking water sampling was conducted at a residential building.

• More than 2.4 billion online plumbing monitoring records were collected.

• Major water pH and organic carbon changes were discovered in the building.

• Seasonal and spatial variations of tap water quality were found across fixtures.

Abstract

Drinking water chemical quality can deteriorate after water enters building plumbing. This study aimed to better understand seasonal and spatial water quality differences in a highly monitored net-zero energy residential building. Water flow rate and temperature were monitored for one year at the service line and at every fixture throughout the crosslinked polyethylene plumbing. Discrete water sampling events (58) were conducted at the service line, 1st floor kitchen sink, 2nd floor bathroom sink, the water heater, and 2nd floor shower. More than 2.4 billion online monitoring records were collected for fixture flow and temperature. In-building water stagnation time varied seasonally and across fixtures. Significant spatial and temporal water chemical quality variations were found. Average seasonal variability was found for service line temperature (15–23 °C) for the total chlorine residual (0.4–0.9 mg/L-Cl₂), NH₃ (<0.01–0.4 mg/L-N), NO₃⁻ (0.1–0.8 mg/L-N), and Cu (32–81 μg/L) concentrations. For 10.3% of the discrete water sampling events, service line water did not contain a detectable total chlorine residual. Water pH consistently and significantly increased in the plumbing system from 7.5 to 9.4, and total trihalomethane (TTHM) levels increased up to 89%. The service line total organic carbon level (0.5–0.6 mg/L) was consistent, but much greater in-building variability was found for cold (0.4–61.0 mg/L) and hot water (0.5–4.7 mg/L). Models are needed to predict chemical water quality at the faucet using service line water quality results and plumbing design and operational information. Building water sensor technology innovations are also needed.

Introduction

In the U.S., 283 million people receive drinking water from a public water system, and while not surveyed, it is assumed this water passes through building plumbing [1]. To reduce the chance that this water can cause disease, a patchwork of federal and state laws restrict the allowable contaminants in water distribution systems [[2], [3], [4], [5]]. Water quality is not monitored at every service line, where water enters the building. Instead, systems must report their drinking water's chemical quality at the point-of-entry and select locations in the water distribution network [[6], [7], [8]]. Compliance with the lead and copper rule is an exception where a water sample is collected from a building faucet [9]. However, this rule does not account for water quality variability during seasons or at all buildings. Thus, information remains limited about the water quality inside these buildings, especially when the degree of water quality may seasonally and spatially differ across building fixtures.

To better understand the factors that influence in-building drinking water chemical and microbial quality, many studies have been conducted [[10], [11], [12], [13], [14], [15], [16], [17]]. The time of day and season can influence service line water quality characteristics like disinfectant residual, water pH, hardness, organic carbon, disinfectant byproduct (DBP), heavy metal concentrations, NO₃⁻ concentrations, and microbial communities [18,19]. Seasonal changes in water temperature and organic content can also influence the rate and type of disinfection by-products (DBPs) formed. A higher water temperature during summer can accelerate DBP formation. The presence of a higher level of dissolved organic carbon (DOC) during summer can also prompt greater DBP levels, and DBPs include trihalomethane (THM) compounds [20,21].

Contaminants in building drinking water can be affected by plumbing materials, temperature, pH, and water flow patterns [22,23]. Temperature influences chemical reaction kinetics; An increase in temperature can increase the disinfectant decay rate, DBP formation, and plumbing component heavy metal leaching [20,21,24,25]. At higher pH values, brass and copper pipes can sometimes leach lower levels of Cu and Zn [16,26], and a greater portion of metals can be in particulate form [27]. A greater amount of THMs was found at higher pH due to the enhanced hydrolysis steps in their formation process [28,29]. The role of pH on organic leaching from plastic plumbing has gone unstudied, but polyethylene resin pellets exposed to pH 11 water released more organic carbon compared to a pH 7.8 water [30]. Changes in water pH can also influence metal precipitation onto crosslinked polyethylene (PEX) plumbing pipes [31]. In-building water stagnation can enable disinfectant residual decay and heavy metal leaching [[10], [32], [33]].

A limitation of prior studies is the lack of intensive water quality monitoring in a single building across seasons and multiple locations in the building. Online monitoring can be costly, requires skilled technicians, and collecting and analyzing water samples is labor-intensive. Online monitoring is commonly used for water source, treatment, and distribution system operations [34,35]. Many prior investigators have collected drinking water from multiple buildings and coupled this with pilot- and bench-scale experiments. To the author's knowledge, no studies have been conducted that (1) utilized online temperature and flow monitoring at every building fixture (cold and hot), with a complete plumbing system layout and accounted for all material types and sizes, and (2) conducted online water quality monitoring at the service line at the building point-of-entry to measure water quality fluctuations.

The study goal was to investigate the temporal and spatial variations of drinking water chemical quality in a highly monitored net-zero energy green building. The authors hypothesized that (1) drinking water chemical quality at the service line will vary seasonally, (2) the duration of fixture (water) stagnation will differ seasonally and spatially, and (3) in-building water temperature, pH, inorganic and organic contaminant levels will vary by season for cold and hot water. To test these hypotheses, water flow and chemical quality monitoring was conducted for an inhabited residential building and bench-scale testing was conducted.