Seasonal lead release to drinking water and the effect of aluminum

Monitoring lead in drinking water is important for public health, but seasonality in lead concentrations can bias monitoring programs if it is not understood and accounted for. Here, we describe an apparent seasonal pattern in lead release to orthophosphatetreated drinking water, identified through point-of-use sampling at sites in Halifax, Canada, with various sources of lead. Using a generalized additive model, we extracted the seasonally-varying components of time series representing a suite of water quality parameters and we identified aluminum as a correlate of lead. To investigate aluminum’s role in lead release, we modeled the effect of variscite (AlPO4 · 2H2O) precipitation on lead solubility, and we evaluated the effects of aluminum, temperature, and orthophosphate concentration on lead release from new lead coupons. At environmentally relevant aluminum and orthophosphate concentrations, variscite precipitation increased predicted lead solubility by decreasing available orthophosphate. Increasing the aluminum concentration from 20–500 μg L increased lead release from coupons by 41% and modified the effect of orthophosphate, rendering it less effective. We attributed this to a decrease in the concentration of soluble (<0.45 μm) phosphorus


Introduction
Lead is a contaminant of concern in drinking water due to its well-documented health effects. 1,2 Many jurisdictions require that it be monitored, but seasonal variation in lead release can bias monitoring programs if it is not understood and accounted for.
Temperature-driven seasonal lead release has been described in previous work, 3 and sampling guidance is often designed to control for temperature effects. 4 But water quality parameters other than temperature can also contribute to seasonal lead concentrations, and aluminum is an important example. Aluminum concentrations can vary seasonally when water is treated to remove particles and dissolved organic matter by coagulation. This is because the solubility of the aluminum hydroxide (Al(OH)3) that precipitates during coagulation with aluminum salts is highly temperature-dependent. Below the solubility minimum at pH 6-7, 5 solubility decreases with increasing temperature, and above pH 6-7, solubility increases with temperature. 6 Treatment facilities that coagulate below the pH of minimum solubility, then, tend to see high residual aluminum in winter, while facilities that coagulate at alkaline pH may see high aluminum in summer. 7 This in turn may influence lead release, but the complex environment of a drinking water distribution system-and the possibility of multiple competing mechanisms-make it difficult to predict aluminum's net effect. Aluminum might precipitate at the scale-water interface as a hydroxide or silicate mineral that slows lead diffusion to the bulk water, [8][9][10][11] but this is controversial. 12,13 It might also precipitate as a phosphate mineral, diminishing the activity of orthophosphate and preventing formation of hydroxypyromorphite (Pb5(PO4)3OH) and other low-solubility phases that control lead release. 5,11,[14][15][16][17][18] Aluminum precipitation that results in suspended particles or colloids may generate a mobile sink for lead, facilitating lead transport from source to tap. [19][20][21] Here, we consider aluminum and other seasonally-varying water quality parameters as drivers of seasonal lead release. We use a hybrid approach that combines statistical analysis of observational data, a factorial experiment, and a mechanistic model. We identify possible origins of periodic lead release in the distribution system of Halifax, a mid-sized North American city, and we isolate a subset of these-aluminum, orthophosphate, and temperature-for investigation using a lead coupon study and a geochemical solubility model. We find that interactions between orthophosphate and aluminum have an important effect on lead release and that variation in aluminum concentrations may play a key role in observed lead concentrations. In our view, mechanisms involving soluble, colloidal, and particulate lead are all relevant to this phenomenon.

Materials and methods
Field sample collection Distribution system monitoring Distribution system samples were collected by utility staff as part of a routine, long-term monitoring program designed to understand the state of the system and respond to water quality issues. Temperature and pH were measured in the field (Hach PH281 probe), and samples were sent to a third-party accredited laboratory for determination of alkalinity, 22 total aluminum, 23 and orthophosphate. 24

Point-of-use corrosion control monitoring
The point-of-use corrosion control monitoring dataset represents two distinct monitoring programs, described in McIlwain 25 and Trueman et al. 26 The first comprised samples collected at residential (1 L volume) and non-residential (0.25 L volume) sites in the distribution system after a minimum 8 hour stagnation period (Table 1). 25 Samples were collected over three years (2010-2012), representing two October and two February collection periods. This program was designed to evaluate the utility's corrosion control program and to identify outlets with high lead levels. The 34 residential sites included three and six with full and partial lead service lines, respectively. A further 18 had copper service lines and the remaining 7 had unknown configurations. Outlets used for drinking or cooking were sampled in 48 non-residential buildings. 25 The second program was designed to evaluate the effect of lead service line replacement on lead levels in tap water. 26 Volunteers residents collected 1 L samples as 4 × 1L minimum 6-hr. stagnant profiles with the addition of a 5-min. flushed sample after each profile (Table 1). We filtered a subset of these using 0.45 µm membrane filters in a syringe-mounted apparatus. To quantify aluminum in the distribution system, we used 5-min. flushed samples only, to minimize the impact of site-specific factors (e.g., premises plumbing). To estimate particulate lead and copper, we used samples collected before replacement, since extreme particulate lead release is typical immediately after replacement.
All samples were collected in high-density polyethylene (HDPE) bottles, cleaned by immersion in ~2 M HNO3 for at least 24 h, and rinsed thoroughly with ultrapure water.

Size-exclusion chromatography
Relative size distributions of lead, aluminum, and iron were determined for a subset of the profile samples described above (see "Point-of-use corrosion control monitoring"), using size-exclusion chromatography with multielement detection (SEC-ICP-MS). The full method is detailed in a previous publication. 20 Briefly, we separated samples on a stationary phase composed of cross-linked agarose and dextran (Superdex 200, 10 × 300 mm, 13 μm particle size, GE Healthcare) with 50 mM tris-HCl (pH 7.3) as the mobile phase. The flow rate was 0.5 mL min -1 and the injection volume was 212 µL. We monitored 27 Al, 56 Fe, and 208 Pb in the column effluent as a function of time by ICP-MS (see Point-of-use corrosion control monitoring above). The retention volume of thyroglobulin (669 kDa, Stoke's radius 8.5 nm), indicated in chromatograms as a qualitative point of reference, was monitored as 127 I. Chromatograms were summarized as the sum of two skewed or exponentially modified Gaussians using the R package fffprocessr, 28 as described elsewhere. 29 R code to reproduce the analysis is included as Supplementary Text S1, the individual chromatograms are shown in Figure S1, and the data are available at doi.org/10.5281/zenodo.5139734.

Lead coupon study
We investigated the effect of three factors-aluminum (0.02 or 0.5 mg Al L -1 ), orthophosphate (0 or 1 mg PO4 L -1 ), and temperature (4 or 21°C)-on lead release from new lead coupons using a set of batch corrosion cells made with new lead coupons.
Using a 2 3 factorial design (Table S1), we evaluated all eight factor combinations (two aluminum concentrations × two orthophosphate concentrations × two water temperatures) to generate independent estimates of each factor's effect, along with estimates of the interactions among factors.

Preparation of test water
Preparation of test water for the coupon study is summarized in Figure 1. We coagulated untreated source water from the water supply plant with Al2(SO4)3 · 18H2O (12 mg Al L -1 ) in a 20 L HDPE plastic container. The coagulant dose was chosen to match the dose applied at the treatment plant supplying the distribution system we studied.
Immediately after adding the coagulant, water was mixed at approximately 800 rpm for one minute using a magnetic stirplate (n.b., rpm is nominal and was determined by the stirplate dial setting). Coagulated water was then mixed for 12.5 minutes each at 600, 500, and 400 rpm. pH was maintained throughout at 6.3 using sodium hydroxide. The flocculated water was allowed to settle overnight, pumped into a separate reservoir, and filtered using a vacuum flask fitted with a 1.5 µm glass-fibre filter membrane. This procedure reduced total organic carbon (TOC) to 1.8 mg L -1 (standard deviation 0.02 mg L -1 ), from an approximate raw water concentration of 3.8 mg L -1 (a summary of untreated water quality is provided in Table S2). TOC samples were collected, headspace-free, in 40 mL clear glass vials and preserved with concentrated phosphoric acid to pH < 2. Vials were washed and then baked at 105°C for at least 24 h before use, and TOC was quantified using a Shimadzu TOC-V CPH analyzer. 30 Filtrate was dosed as needed with H3PO4, Al2(SO4)3 · 18H2O, and NaHCO3 (5 mg C L -1 ) to achieve the experimental conditions listed in Table S1. The initial pH for all test waters was adjusted to 7.5 with HNO3 and NaOH. pH was measured using a combination electrode, and the nominal orthophosphate concentration was verified colorimetrically. 24

Coupon conditioning
Corrosion cells were refilled with 50 mL of fresh test water according to the experimental design summarized in Table S1; this volume was chosen to prevent contact with the sealant while minimizing headspace. We completed 42 changes of water before beginning to collect data, and each change was followed by a minimum 24 hour stagnation period. After conditioning, lead in 0.45 µm filtrate agreed reasonably well with predicted equilibrium lead solubility, with a mean absolute error of 8 µg L -1 at the low level of aluminum, a temperature of 21°C, and either 0 or 1 mg PO4 L -1 .

Sample collection
After each 24-hour stagnation period, cells were mixed by inverting five times. Aliquots of 10 mL were then decanted into polypropylene tubes, acidified to pH < 2 with concentrated trace metal grade nitric acid, and held for a minimum of 24 hours before analysis. Separate 10 mL aliquots were filtered, immediately after collection, using 0.45 µm membrane filters in a syringe-mounted apparatus.

X-ray diffraction
We identified crystalline phases in coupon corrosion scale using X-ray diffraction (XRD).
Coupons were dried and analyzed without removing scale from the surface. We used a Rigaku Ultima IV X-ray diffractometer with a copper K radiation source, operated at 35 kV and 30 mA. Scans were acquired over the range 10-70° (2 ) with a step size of 0.04° and a scan speed of 0.8° min -1 . The powder diffraction file numbers corresponding to standards referenced in the manuscript are listed in Table S3.

X-ray photoelectron spectroscopy (XPS)
The elemental composition of corrosion scale was determined by XPS using a Thermo VG Scientific Multilab 2000 instrument. An aluminum X-ray source was used under a high vacuum, and a CLAM4 Hemispherical Analyzer with a multichannel detector was used to detect photoelectrons. Survey scans were acquired at a pass energy of 50 eV with a step size of 1.0 eV, and high-resolution scans were acquired at a pass energy of 30 eV with a step size of 0.1 eV. Binding energy was calibrated using the C 1s spectral line, due to adventitious carbon, at 285 eV.

Data analysis
We used R for data analysis and visualization, 31 along with a collection of widely used contributed packages. [32][33][34][35] Paired comparisons of lead levels at the point of use Paired measurements collected at the point of use in October and February were compared using a parametric test of mean difference for censored data, using the cen_paired() function in the NADA2 package. 36 (Censoring here refers to lead concentrations below the reporting limit.) Duplicate measurements at sites within a single group were averaged; when one was observed and one censored, the duplicate measurements were re-censored at the midpoint value. Due to a log transformation of the data, back-transformed group differences are expressed as ratios. R code required to reproduce the analysis is provided as Supplementary Text S2, and data are available at doi.org/10.5281/zenodo.5139734.

Equilibrium lead solubility modeling
We modeled equilibrium lead solubility using tidyphreeqc, 37 an R interface for PHREEQC, 38 and pbcusol, 39 an extension of tidyphreeqc. Thermodynamic data relevant to the lead-water-carbonate-orthophosphate system were sourced from Schock et al. 40 (Table S4), and activity coefficients were calculated as described in Parkhurst and Appelo. 38 Model inputs were pH, orthophosphate, and dissolved inorganic carbon concentration, calculated from pH and alkalinity. 41 We assumed that lead solubility was controlled by hydroxypyromorphite, a mineral that has been identified in lead pipe corrosion scale recovered from the distribution system described here. 42 While polyphosphate-dosed at the treatment plant at approximately 0.04 mg P L -1 -is known to form soluble complexes with lead, we did not identify lead-polyphosphate complexation at the point of use via SEC-ICP-MS. 20,42 For this reason, we opted not to include polyphosphate in the solubility model. (Lead-polyphosphate complexes were evident by the same method in another distribution system, at a higher polyphosphate dose of approximately 0.2 mg P L -1 . 42 ) Since there were not enough paired distribution system data to include aluminum in the model, we fit a separate model to account for aluminum's effect. We calculated hydroxypyromorphite solubility on a grid of orthophosphate and aluminum concentrations at pH 7.5 and 5 mg L -1 of dissolved inorganic carbon, assuming that both hydroxypyromorphite and variscite (AlPO4 · 2H2O) reached equilibrium with the solution.
Thermodynamic data describing variscite dissolution and two aqueous aluminum phosphate species were sourced from Roncal-Herrero and Oelkers, 43 and R code to reproduce the analysis is included as Supplementary Text S3.

Distribution system monitoring data
We fit generalized additive models (equations 1-2) 44 Generalized additive models included a multi-year trend, a seasonal trend, and an autoregressive error term. 46 The multi-year trend was estimated using a thin plate regression spline and the seasonal trend using a cyclic cubic regression spline. 44 We fit separate cyclic splines to orthophosphate data collected at the treatment plant and in the distribution system, and we included a parametric term to model the difference in orthophosphate residual between these two groups. The autoregressive error term was second order in the models fitted to the temperature and orthophosphate product dose series and continuous-time first-order otherwise. Equation (1) describes the basic model.
In equation (1), is the response, 1 is the numeric date, 2 is the day of the year, 0 is the intercept, is the error term, and the ( ) are linear combinations of basis functions (equation 2). ( In equation (2) Table S5; residuals were approximately Gaussian ( Figure S5), homoscedastic ( Figure S6-S7), and largely free from autocorrelation ( Figure S8).

Static corrosion cell data
We fit a linear regression model to the 2 3 factorial coupon study after a natural log transformation of the response, as described in Montgomery. 47

Results and discussion
Aluminum and orthophosphate seasonality in the distribution system Aluminum levels were strongly seasonal in the distribution system we studied ( Figure   2). Median aluminum was highest in February and lowest in July: 182 and 32 µg L -1 respectively. The aluminum residual in treated water is generally highest when water temperature is lowest, 21 due largely to the inverse temperature dependence of aluminum hydroxide solubility at the median coagulation pH of 5.75. 6,48,49 Median water temperature in these two months was 5 and 20°C. Orthophosphate also exhibited a seasonal pattern. This is due primarily to variation in the applied corrosion inhibitor dose ( Figure S10), but seasonal variation in the reversion rate of polyphosphate may have also been a factor. 50 Minimum and maximum orthophosphate concentrations occurred in February and May respectively (130 and 170 µg P L -1 ), approximately opposite those of aluminum (Figure 2b). Orthophosphate was 11% lower in the distribution system compared to the treatment plant, as estimated by a parametric term in the generalized additive model (Table S5). Aluminum precipitates with orthophosphate as AlPO4, 5 which may contribute to this difference and to the seasonal pattern in the distribution system. Alkalinity exhibited a bimodal seasonal pattern, with maxima in March and July and a minimum in December, while seasonal maximum and minimum pH occurred in September and February, respectively.

Seasonal variation in predicted equilibrium lead solubility
Variation in orthophosphate, pH, and alkalinity predicted a complex seasonal pattern in equilibrium lead solubility, with two prominent peaks (Figure 2b). The first occurred in

Periodic variation in lead at the point of use
Consistent with equilibrium solubility predictions, lead release exhibited periodicpossibly seasonal-variation concurrent with that of aluminum and opposite to that of orthophosphate. We compared lead levels in first-draw samples collected in October with those measured in February at matched sites and drinking water outlets ( Figure 3).
Lead release to standing water in October was an estimated 65% of that in February (p <<0.001, n = 134, signed-rank test). Copper release exhibited a similar trend: its concentration in standing water in October was an estimated 67% of that in February (p <<0.001, n = 134). While these data represent total concentrations only, lead and copper concentrations in 0.45 µm filtrate were an estimated 75 and 89% of the corresponding total concentrations in paired aliquots representing 360 samples collected as profiles from residences with full or partial lead service lines (as described in Trueman et al., 26 Figure S11). This suggests that lead and copper were largely present in the system in forms smaller than 0.45 µm.
On a percentage basis, differences in lead release were larger than expected based on lead solubility-predicted equilibrium lead concentrations were just 8% lower in October compared with February (accounting for variation in pH, alkalinity, and orthophosphate).
This discrepancy suggests that factors not captured by the solubility model-processes involving aluminum, for instance-were important. Observed differences were probably not due to water temperature: during overnight stagnation seasonal temperature variation is significantly damped, 51   Relative size distributions of lead, aluminum, and iron were typically bimodal ( Figure   4a), with two incompletely resolved peaks representing colloids with different apparent molecular weights. Aluminum co-occurred with lead (and iron) in at least one of these two fractions in all samples with detectable aluminum peaks (Figures 4a and S1). This is consistent with previous work documenting adsorption of lead to aluminum hydroxides [56][57][58] or mixed iron/aluminum (oxyhydr)oxides, 59 and with previous studies reporting occurrence of lead and aluminum in a common colloid size fraction. 20,54,55 The presence of aluminum, iron, and lead in distinct but overlapping colloid populations, however, cannot be ruled out completely. Moreover, these data do not provide a complete picture of colloid composition; the role of phosphorus, for example, is not clear.

Interaction between aluminum and orthophosphate (lead coupon study)
Distribution system monitoring data suggest that variation in both aluminum and orthophosphate may have contributed to the seasonal differences in lead release, but it is not clear which factor was more important or to what extent they acted synergistically.
We evaluated these factors-along with water temperature-as predictors of lead release using a coupon study. While the effect of orthophosphate on equilibrium solubility is relatively well understood, its interactions with other species to form particles are less well characterized. 60 As expected, lead release from coupons increased with water temperature. Raising the cell temperature from 4 to 21°C caused a 120% increase in geometric mean lead release (Figure 5a).  Adding 1 mg PO4 L -1 decreased total lead release by 34% (Figure 5a), while aluminum had the opposite effect; increasing the aluminum concentration from 20 to 500 µg L -1 increased total lead release by 41%. Adding orthophosphate and increasing aluminum concentration accompanied a further 61% increase in lead. That is, the combined effect of aluminum and orthophosphate was larger than would be expected based on the main effect of each factor. This may be due to formation of particulate aluminum and phosphorus-perhaps as aluminum phosphate. Particulate phosphorus was highest at the high aluminum level, and in this form it would presumably be less available to react with lead in a way that immobilized lead at the scale-water interface (Figure 5c and Figure S12). Consistent with this interpretation, substantially less phosphorus was lost to the system at the high aluminum level (i.e., more remained in the water phase).
Particulate lead was also greatest at the high aluminum and orthophosphate levels ( Figure 5c), which may be due to partitioning of lead to precipitated aluminum phosphate.
With lead in 0.45 µm filtrate as the response, several effect estimates in the linear model were notably different. Adding orthophosphate, for instance, caused a much larger percentage decrease in filtrate lead levels (78%, Figure 5a). This is consistent with orthophosphate's expected effect on lead solubility, while effective control of particulate lead by orthophosphate requires that lead phosphate precipitates become immobilized in corrosion scale. Here, P:Pb molar ratios were much greater than one, a threshold that has been noted previously to promote formation of dispersed lead phosphate particles. 60 And in addition to the effect of aluminum in boosting particulate lead concentrations, the dispersive effect of orthophosphate may be especially pronounced at the relatively low hardness characteristic of our experimental water (3.9 mg CaCO3 L -1 , Table S2). Orthophosphate-induced dispersion is also enhanced in the presence of humic and fulvic acids. 60 And while coagulation here would have removed the majority of the hydrophobic acid fraction, 62 natural organic matter may still have played a role in dispersing particulate lead. 63 In contrast to its effect on total lead release, aluminum decreased lead in filtrate by 21% ( Figure 5a, neglecting the aluminum-orthophosphate interaction). This agrees with previous work suggesting that aluminum may promote formation of a diffusion barrier on lead composed of aluminum hydroxide, silicate, or other compounds. 11 Alternatively, aluminum may have facilitated partitioning of soluble lead to suspended particles, shifting the size distribution of lead in the test waters.

Coupon corrosion scale
We characterized the corrosion scale that formed on coupons under all experimental conditions using XRD (Table S1)  Aluminum was not identified in any crystalline mineral forms by XRD, and the experimental patterns representing coupons exposed to 20 and 500 µg Al L -1 were similar ( Figure 6). Moreover, aluminum was not detectable by XPS in the top few nanometers of corrosion scale exposed to the high level of aluminum (0.5 mg Al L -1 ) ( Figure S13). Thus it is likely that the aluminum content of scale was relatively low, although XPS detection limits for light elements (e.g., Al) in a heavy element matrix (e.g., Pb) tend to be above 1 atomic percent. 64 The low surface concentration of aluminum is consistent with our interpretation that aluminum acted primarily by promoting particulate lead formation and limiting the activity of orthophosphate in solution. Moreover, the mineralogy of the scale, as determined by XRD, was predictable without considering the aluminum concentration.
On the longer time scales relevant to drinking water distribution, however, aluminum may alter lead corrosion scale in a way that impacts lead release. Here, the apparent effect of aluminum in limiting dissolved lead release in the absence of orthophosphate was relatively small, and it was not due to readily discernable differences in coupon scale at the high and low aluminum levels.

Modeling aluminum-phosphate interactions
Key findings from the coupon study-high lead release from and inhibited phosphorus uptake by corrosion scale in the presence of aluminum-agree well with a previous report that aluminum interferes with orthophosphate corrosion control. 14 Given the results we report, this is likely due to both increased solubility and particle-generating mechanisms. And while the full picture is complex, the effect of aluminum on lead solubility-neglecting particles and surfaces-can be modeled by allowing coprecipitation of aluminum and orthophosphate (here as variscite, AlPO4 · 2H2O) in the presence of hydroxypyromorphite (Figure 7). We applied this model over a grid of aluminum and orthophosphate concentrations (Figure 7a and b) and, neglecting other sources of variation, to the aluminum concentrations measured in the distribution system ( Figure 7c). Consistent with the experimental results, aluminum phosphate precipitation increased lead solubility by decreasing the concentration of orthophosphate in solution. This was predicted to occur except at very low aluminum concentrations (e.g., approximately 50 µg Al L -1 at 1 mg PO4 L -1 , Figure 7b). And at the aluminum concentrations characteristic of the distribution system, significant seasonal variation in lead solubility is predicted (Figure 7c).

Conclusions
We identified an apparent seasonal pattern in lead release to orthophosphate-treated drinking water via point of use sampling. And while variation in orthophosphate, pH, and alkalinity predicted a similar pattern in equilibrium lead solubility, seasonal variation in aluminum may have also been a factor, given its strong correspondence with observed lead levels. In a follow-up coupon corrosion study, aluminum increased total lead release significantly. As expected, orthophosphate decreased lead release, but high levels of aluminum and orthophosphate together resulted in greater lead release than would be predicted based on the main effects of these two factors. We suggest that the interference of orthophosphate corrosion control by aluminum is due largely to precipitation of aluminum phosphate. This reaction limits the activity of orthophosphate and may provide a surface to which soluble lead can partition-increasing the total lead content of drinking water.
Our data suggest that treatment facilities applying aluminum-based coagulants should ensure that residual aluminum in treated water remains low to limit seasonal variation in the performance of orthophosphate. In the water system we studied, a recent increase in coagulation pH to 6.2 has decreased the median April aluminum concentration at the treatment plant by a factor of more than four relative to the 2003-2016 study period. At the more recent concentrations, predicted aluminum phosphate precipitation is minimal (<1 µmol), even at a higher orthophosphate dose of 1 mg PO4 L -1 . The predicted effect of aluminum on equilibrium lead solubility, then, is also much smaller.
More generally, aluminum-orthophosphate-lead interactions highlight an important connection between corrosion control and the treatment process, potentially involving the soluble, colloidal, and particulate fractions of these elements.

Supporting information
Individual SEC chromatograms; details of the generalized additive models fit to water quality time series; additional desription of the coupon study; R code to reproduce the water quality data analysis and the geochemical models.             Figure S13. High-resolution XPS scans of lead coupons exposed to 0.5 mg Al L -1 , before and after removal of corrosion scale. The feature at 73 eV was present in scans acquired before and after removal of the corrosion scale and was not attributable to aluminum. Tables   Table S1. Static corrosion cell experimental design.

Supplementary information for Seasonal lead release to drinking water and the effect of aluminum
[Al] represents observations in 24-hour stagnant test water, while the other settings are nominal.  Table S2. Chemical analysis of untreated water from the water supply plant (source: utility data).