WaterSubject

The mobility of phosphorus (P) in septic system plumes remains a topic of debate because of the considerable reactivity of this constituent. In this study, a septic system plume in Ontario was monitored over a 16-year period with detail that clearly shows the advancing frontal portion of the P plume. This monitoring record provides insight into the extent of secondary P attenuation in the ground water zone beyond that available from previous studies. A P plume 16 m in length developed over the monitoring period with PO₄-P concentrations (3 to 6 mg/L) that approached the concentrations present under the tile bed. Simulations using an analytical model showed that when first-order solute decay was considered to account for the possibility of secondary P attenuation in the ground water zone, field values could only be matched when decay was absent or occurred at an exceedingly slow rate (half-life greater than 30 years). Thus, hypothesized secondary P attenuation mechanisms such as slow recystallization of sorbed P into insoluble metal phosphate minerals, diffusion into microsites, or kinetically slow direct precipitation of P minerals such as hydroxyapatite were inactive in the ground water zone at this site or occurred at rates that were too slow to be observed in the context of the current 16-year study. Desorption tests on sediment samples from below the tile bed indicated a PO₄ distribution coefficient (K_d) of 4.8, which implies a P retardation factor of 25, similar to the field apparent value of 37 determined from model calibrations. This example of inactive secondary P attenuation in the ground water zone shows that phosphorus in some ground water plumes can remain mobile and conservative for decades. This has important implications for septic systems located in lakeshore environments when long-term usage scenarios are considered.

Laboratory column and batch studies investigating phosphorus (P) mobility in sediments invariably note retardation of P due to surface sorption processes, but in most cases also note additional loss of P mass and a tendency for increasing irreversibility of P sorption with time. This behavior has been attributed to the presence of multiple pools of sediment P, including a labile pool, which is easily desorbed, plus other secondary pools that are more stable. Several processes have been suggested to account for the stable secondary P, including molecular diffusion of P into micropores or through mineral coatings onto internal sorption sites, slow crystallization of sorbed P into insoluble metal phosphate minerals, or direct precipitation of metal phosphate minerals.

The occurrence of secondary attenuation has important implications for the fate of P in ground water plumes, particularly those generated by septic systems. In a review of PO₄ behavior in 10 well-characterized septic system plumes in Ontario, six sites on sands had distinct PO₄ plumes more than 10 m in length, with PO₄-P concentrations (1 to 6 mg/L) that were orders of magnitude higher than threshold levels that could stimulate algal growth and eutrophication in lakes. Although P migration in septic system plumes is always highly retarded because of sorption processes, typically by a factor of 10 to 100 in sand aquifers, the degree of reversibility of P sorption reactions in field settings has not been well established. The distinction between normal fast reversible sorption and other secondary processes is important because reversible sorption does not permanently remove P from solution and consequently this P mass remains ultimately mobile and conservative in the flow system. On the other hand, if secondary processes are active, even at a slow rate, they could be of considerable importance in immobilizing P mass when considering the decade-scale time frames normally associated with septic system use and the slow migration rate of PO₄.

In our previous review of P mobility in septic system plumes, field evidence showed that substantial P mass removal (25% to 99%) occurred within the first 1 to 2 m of subsurface flow, largely coincident with the unsaturated zone. This was the result of rapid mineral precipitation reactions prompted by redox changes occurring within this zone. Additional declines in P concentrations were also noted farther along the plume flowpaths. However, the previous studies were not sufficiently detailed to determine if this was the result of secondary attenuation or other processes such as hydrodynamic dispersion which could produce a similar effect.

Compared to laboratory studies, long-term monitoring of field-scale plumes should provide a better opportunity to ultimately establish the importance of secondary P attenuation processes. However, few such studies have been undertaken. One site where long-term monitoring of a ground water P plume has been undertaken is at the Massachusetts Military Reserve on Cape Cod, Massachusetts. Here, a P plume approximately 600 m in length emanates from a set of large sewage infiltration beds that have been in operation since 1936. Detailed monitoring of the P plume was initiated in 1993, at which time the plume was already well established and was discharging to a small kettle lake (Ashumet Pond). Variations in PO₄ concentrations along the length of the plume, in part, reflect irregular sewage loading during earlier decades. Additionally, changes in redox conditions along the plume cause changes in the concentrations of Fe and Mn, which in turn affect saturation conditions with respect to P minerals such as strengite and vivianite. This further complicates the interpretation of P trends at the site. Both one-dimensional (1D) streamtube modeling and multidimensional reactive transport modeling incorporating field P mass distribution and plume geochemistry have been undertaken in an effort to predict future P loading to the lake.

The current study was initiated specifically to address the possibility of secondary P attenuation in septic system plumes. One of the sites described previously in our review paper (Long Point, tile bed 2) was selected for this purpose because of several considerations. A detailed monitoring network was installed at this site prior to commissioning of the tile bed in 1990; thus, the complete history of sewage loading and P plume development over a 16-year period is available. In 2003/2004, the existing monitoring network was augmented with eight additional multilevel piezometer bundles targeting specifically the advancing frontal portion of the P plume. The septic system is a large flux system servicing a seasonal use campground and generates a plume that is easily distinguished from the relatively pristine background ground water at the site. The plume occurs within a moderately homogeneous calcareous sand aquifer that has a limited dispersive capacity; thus, the plume retains a large core zone that is unaffected by dilution. In the plume area where PO₄ is present, NO₃-N concentrations are consistently greater than 10 mg/L and Fe remains less than 0.1 mg/L. Thus, the P plume remains within a single redox environment and the complexities of P fate at redox boundaries, particularly when Fe concentrations change, are avoided. Additionally, the seasonally intermittent nature of the sewage loading at this site provided a mechanism to directly measure the bulk horizontal ground water velocity, which in many studies is difficult to measure.

The 16-year monitoring history at this site and the sampling detail that shows the advancing frontal portion of the P plume are not available in previous studies. This evidence is crucial in establishing the importance of secondary P attenuation, particularly if these reactions occur slowly.

The phenomenon of secondary slow or irreversible sorption of P widely reported in laboratory studies was not evident during long-term monitoring of P migration in the Long Point septic system plume. None of the processes suggested to account for secondary attenuation, such as P diffusion into soil particle microsites, slow recystalization of sorbed P to insoluble metal phosphate minerals, or kinetically slow direct precipitation of P minerals, was active enough to be observable in the ground water zone at this site.

A phosphorous K_d value of 4.8 was obtained from desorption tests on core samples from the source area. Use of this value with the well-known retardation equation, which assumes that fast reversible sorption is the only attenuation mechanism operating, provided an accurate prediction of the P migration rate at the site. This success opens up the possibility of using simple field tests to provide estimates of P migration rates at septic system sites.

The absence of an “irreversible” P attenuation mechanism at this site has important implications for the long-term fate of P in septic system plumes. Without a secondary process, P in the ground water zone remains ultimately mobile, and thus a downgradient surface water course, in this case, Lake Erie, will eventually be impacted. Although hydrodynamic dispersion may dampen peak concentrations arriving downgradient, dispersion by itself cannot lower mass loading rates. The modeling exercise did demonstrate, however, that even very slow secondary attenuation, beyond that which might be evident in the context of the current 16-year study, could still provide important P mass reductions over longer travel distances. Further research should continue to investigate this possibility.

Similar comprehensive observations of P plume advancement have not been presented in previous studies, with the possible exception of the Cape Cod site. Nonetheless, the Long Point plume is typical of septic system plumes generated by domestic waste water, and results should thus be applicable to many other sites where properly functioning septic systems occur in sandy calcareous terrain.