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TP/Cha Predictive Models

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: May 13, 2020
© S.M. Mandaville Post-Grad Dips. (lakescienceoutlook.com)

The citation for this documentation is:

Mandaville, S.M. TP/Cha Predictive Models. Nova Scotia, Canada. Electronic media.

Introduction

See also Selection of Phosphorus Loading Model for Nova Scotia- Phase I.

Predictive TP modelling was based primarily on the following: extensive decadal research spearheaded by several international peers in limnology under the chairmanship of Richard A. Vollenweider PhD, formerly of the Canada Centre for Inland Waters, Environment Canada, which culminated with the consensus OECD (1982)- Organisation for Economic Co-Operation and Development Report; on the Vollenweider(1976) TP Predictive Model; and the published research of Peter J. Dillon FRSC et al., formerly of the Dorset Research Centre, Ontario Ministry of the Environment.

In addition, Mandaville has been incorporating other regressions published in the latest international limnological journals as deemed necessary. This process will take several years for model revisions as well as calibrations.

The natural background (+ direct aerial deposition) concentrations were computed utilizing published export coefficients in generally undisturbed similar natural soils in Nova Scotia (cf., Hart et al, 1978; Mandaville, 2000; Scott et al, 2000; Scott et al, 2003); the post-development concentrations were obtained utilizing published export coefficients (end-of-pipe mean year averages) of typical residential and commercial/institutional developments (cf., Mandaville, 2000; Scott et al, 2003; Vokey, 1998; and typical pollutants in stormwater runoff).

With respect to areas served by septic systems, our revised models did not incorporate the same assumptions as in Hart et al (1978) and Scott et al (2003) where the authors assumed 50% septic-derived phosphorus retention within 300m of lakes in the Halifax/Wolfville soils. In the Scott et al (2003) report where Mandaville was a co-author, there was an inadvertent omission made by not noting this down (and Mandaville regrets that). Our export coefficients in such areas include all potential sources of phosphorus inputs incorporated into the 50% export assumption; it actually varied from 50% in some watersheds based on local info.

As a worst case scenario, 0% retention may be assumed for the long term as per the Province of Ontario's guidelines, "Protecting Water Quality in Inland Lakes on Ontario’s Precambrian Shield" (2010).

Table: Trophic characterisation of lakes impairment of various uses
(Vollenweider and Kerekes, 1982)
Limnological characterisation	Oligotrophic	Mesotrophic	Eutrophic
General level of production ........	low	medium	high
Biomass ............	low	medium	high
Green and/or blue-green algae fractions	low	variable	high
Hypolimnetic oxygen content .....	high	variable	low
Impairment of multi-purpose use of lake	little	variable	great

Predictive graphical models utilized

Predictive graphical models are constructed with the EasyPlot software utilizing the data in our MS Excel spreadsheet models, the latter being `dynamic'. The world renown and internationally used base models are the Organization for Economic Co-Operation and Development (OECD)'s Management Model, and the Vollenweider (1976) Model. These graphs visually depict the modelled theoretical as well as any relevant `recent' field values, hence, serve as superb bases.

The OECD (1982) Management Model (Vollenweider and Kerekes, 1982):

This model synthesizes the standard OECD equations for the relationships between average inflow phosphorus concentration (P_j), expected average lake concentration P_λ, and expected average chlorophyll concentration (Cha) as a function of the average water residence time T(w). The model also gives approximate indications of the expected trophic category.

As these categories are management oriented, they are slightly more stringently defined (i.e., approximately at the class midpoints) than are the categories used for diagnostic purposes. This provides a certain safety margin for the design of the loading objectives. Since the model requires the hydraulic residence time as one of the axes, it is not possible to plot most lakes due to insufficient bathymetric data.

OECD/PIC/Table 2-4.jpg

The Vollenweider (1976) Model:

Although this model pre-dates the above OECD (1982) Model, in most cases it is more appropriate since one of the axes-variables, qs (areal water load) is available for all lakes. In order to incorporate the probabilistic scenario, the management categories from the OECD (1982) model have been incorporated into this model. When possible and relevant, lakes with sufficient bathymetric data were plotted on both of the above models.

Nomenclature:

Background -plus- aerial deposition (B+A):

The theoretical background loading -plus- direct aerial deposition have been noted as "Th B+A", and in clearwater lakes it is expected that these were the natural background values including direct aerial deposition.

Recent development scenario:

Onsites- those existing within 300 m of all watercourses inclusive of upstream areas (Dillon and Rigler, 1975; Dillon et al., 1986; and Dillon et al., 1994).
Urban/Serviced- existing developed areas in the watersheds with sanitary sewerage exported out of the watershed. Sewage Treatment Plant (STP) contributions taken into account.

Future development scenario:

Onsite disposal systems:: F-P (Future Probable)= lands within 300m of all lakes developed with onsite systems @ 2.5 lots/ha density.; F-U (Future Ultimate)= F-P scenario + lands within 300m of all streams developed with onsite systems @ 2.5 lots/ha.
Future Sewage Treatment Plant (STP) contributions taken into account.
Urban/Serviced (watersheds developed as residential- sanitary sewerage exported out of the watershed):: F-P (Future Probable)= Urban/serviced area export coeff. @ 0.52 kg/ha.yr (Waller, 1977; and Vokey, 1998).; F-U (Future Ultimate)= Urban/serviced area export coeff. @ 1.1 kg/ha.yr (Waller and Novak, 1981).

No scientifically based allowance was made to take into account the highly biological nature of the effluent from STPs, although the lower removal efficiencies from Myers and Harding (1983) were applied when possible.

Notes of caution:

Contribution from humans to sanitary sewage systems- Our original models assumed 0.8 kg/capita/yr TP based on the Ontario models from the 1970's to the 1990's. Ontario reported lower values of 0.66 kg/capita/yr in 2006 due primarily to decreases in the phosphorus content of detergents. We have now updated most predictive models in Excel. Though, we have not revised the graphical models, but the pre-cultural values would not change.
Regarding pets (dogs and cats), the contribution can be computed based on the relative body weights in comparison with an average human being. Most of this would be contributed via stormwater inputs.
For contributions from waterfowl, peruse our submission to the Halifax Watershed Advisory Board in year-2000.
The predictive modelling does not take into account potential large inputs of phosphorus from failing pumping stations, bypasses from treatment plants/pumping stations, cross-connections between sanitary and storm sewers, and other unforeseen sources. But if a value is known, it can easily be added in the `miscellaneous' column of the control spreadsheets.

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