A field study assessing the impact of on site valerie mc-carthy_slideshare

A field study assessing the
impact of on-site wastewater
treatment systems on surface
water quality in a Co. Monaghan
catchment

Valerie McCarthy, DkIT

International Symposium On Domestic Wastewater Treatment
& Disposal Systems
Trinity College, Dublin, September 2012

www.valeriemccarthy.me

Background Collected Data Study Aims Site Summary Results Conclusions

Study Site Location
Milltown Lake is an
abstraction point for the
Churchill and Oram Group
water Scheme
Services 1,922 people
Approximately 735 m3 of
water being abstracted
daily


Milltown Lake catchment
Monitoring has taken place within Milltown Lake
catchment for point & diffuse sources of
contamination
Identifying the various locations, land uses &
activities, which are likely sources of
contamination
Actively engaging community support,
commitment & participation through inclusion
at all stages of the process


Milltown Lake catchment
Pressures to water quality identified within the
catchment include:
Poor farmyard practices
Direct access of cattle to water courses
Run-off from slurry & fertiliser
spreading

Geological Survey of Ireland
(GSI) subsoil permeability map
records the subsoil in the general
area as of ‘low’ permeability,
suggesting that runoff usually
dominates over infiltration.


Ability of OSWTS at effectively
treating effluent is dependent on:
Soil type:
Grain size
Chemical composition
Depth to bedrock
Proximity to surface water
Potential presence of preferential flow path
Capacity of the system in relation to the number
of people using it
Chemical composition of the effluent received by
the system
Frequency at which the tank is de-sludged
Correct functioning of components of the system
itself
Depth of unsaturated subsoil layer


Collected data
OSWTS were also identified as a potential
source of contamination within the
catchment
Consequently they were characterised
through a combination of:
Household questionnaires
Non-intrusive site inspections
Intrusive monitoring programme


Household Questionnaire

Total of 154 households
surveyed
Average number of
occupants per house = 3.1
91 % of the systems were
septic tanks
27 % of the respondents
never de-sludged their tank

Over half of the systems > 16 yrs old


Non-intrusive survey
A subset of 42 systems were visually inspected
83 % of which were septic tanks
45 % had percolation areas
38 % of systems relied on soakways (pits filled with stones)


Non-intrusive survey
17 % of systems which direct discharges into drains (two of which had
no settlement tanks at all)


Intrusive Study
Five sites were selected for intrusive
investigation
Varying numbers of piezometers were
installed down-gradient of each system
depending on proximity to water courses &
one control located outside predicted area of
plume
Samples collected once every two weeks
between Aug 08 – Aug 09 from each
piezometers and from nearest water course
– up & down-stream from the OSWTS
Analysed for a range of biological &
chemical indicator parameters


Aims & Objectives
This project focused on systems currently in use, regardless of age,
type or level of maintenance & aimed to:

Investigate the fate of contaminants arising from OSWTS
To provide quantifiable data on the contribution of OSWTS to
nutrient loading within the catchment
Possible effects of factors such as poor maintenance, incorrect
installation & operation & location in areas with unsuitable
site characteristics were taken into account


Site summary

System Dispersal Distance to No. of Age De-sludging
method watercourse Residents (yrs)
(m)

Site D Secondary Percolation ~ 102 2-4 ~5 Every 2 yrs

Site F Septic Tank Soakaway ~ 44 2-3 ~ 35 Every 1 yr

Site K Septic Tank Soakaway ~ 12 1-3 ~ 30 intermittently

Site J Septic Tank Soakaway ~ 15 2 ~ 35 intermittently

Site S Septic Tank Percolation >200 2 ~ 30 intermittently


Site characterisation
Site characterisation carried in Nov 2008 (EPA, 2000)
Incorporated a percolation test to ascertain the
assimilation capacity of the subsoil

T value P value
min/25 mm min/25 mm
None of the sites were deemed
Site D 38 23
suitable for a conventional septic
Site F 98 25 tank system
Site S > 100 63 Two were considered suitable for
Site K
the installation of an advanced
> 82 60
wastewater sewage treatment
Site J > 90 65 system by the assessor, both with
Site S >100 63 recommendations for a soil polishing
filter
EPA (2000) Wastewater Treatment Manuals: Treatment Systems for Single Houses. Environmental Protection Agency


Water Table Levels
Depth to Water Table The water table was relatively stable
throughout the monitoring period
Depth to Water Table
The combined effects of a shallow
(m.b.g.l.)
water table and a thick capillary
Site D
fringe at these sites indicate a
0.15 - 2.65
reduced unsaturated zone
Site F 0.18 – 2.9
Site K 0.37 – 2.77
Site J 0.77 – 1.37
Site F 0.12 – 0.93

Number of issues relating to installation
of systems were observed following site
assessment


Example of Site Layout (Site D)


Example of Site Layout (Site F)


Subsurface Water Quality
Distribution of chloride
(Cl-) concentrations,
Site F, Aug 08 – Aug 09


Distribution of ammonia (NH3+-N) and nitrate (NO3--N) conc, Site F, Aug
08 – Aug 09


Mean values of indicator parameters up- and down-gradient along the
effluent plume path, Site F and D, Aug 08 – Aug 09
Site F Site D
Up- Up-
Down-gradient Down-gradient
gradient gradient
BHF2 BHF8 BHD4 BHD9
Distance from Tank
(6.4 m) (48 m) (22 m) (102 m)

DIN
(mg L-1-N)
46.7 9.57 128.2 8.11

DOC 16.5 2.4 29.1 2.6
(mg L-1)
SRP
(PO43--P mg L-1)
0.02 0.03 27.6 0.02
TC
96 - 397,260 0 - >2,420 18. - 241,960 0 - >2,420
(MPN/100 ml)

E. coli
1 - 310,620 0 - >2,420 0 - 11,190 0 - 365
(MPN/100 ml)


Direct Discharges

Site K Direct Discharges

Accordingly, the principal pathway of potential contaminant delivery to
nearby watercourses is unlikely to have been via subsurface flow

Site J Direct Discharges


Direct Discharges
Preliminary investigation of direct discharge
points:

Sampling carried out at 4 discharge pipes
Two pipes were of unknown origin
Monitored for the presence of caffeine &
other indicator parameters

Caffeine was detected at two of the sites:
Site K and one unknown (Site TH5)
A linear relationship was observed between
E. coli and caffeine at these sites r2=0.95


Direct Discharges
Caffeine was detected at two of the sites:
Site K and one unknown (Site TH5)
A linear relationship was observed between
E. coli and caffeine at these sites r2=0.95

Faecal bacteria, Site K & TH5, Feb 12
E. coli TC
(MPN/100 ml) (MPN/100 ml) E. coli, Site K, Feb 12
Min Max
Site K 261.3 - > 2419.6 >2419.6
MPN day-1 MPN day-1
TH5 64.4 – 1,553 >2419.6
1,960 4.8 x 105


Surface Water Quality
Summary of surface water quality adjacent to sites

NH3+ (mg L-1- N) 0.08 ± 0.01

NO2-(mg L-1- N) 0.04 ± 0.009

NO3-- (mg L-1- N) 1.13 ± 0.1

SRP (PO43-- mg L-1-P) 0.01 ± 0.001

TP (PO43-- mg L-1-P) 0.02 ± 0.002
Total Coliforms
10 – 24,196
(MPN/100 ml)
E. coli
(MPN/100 ml) 0 – 17,329


Conclusions
Parameters such as P were largely removed during the soil treatment
process
Some, such as indicator bacteria also experienced large reductions,
nevertheless, max total coliform >2,420 MPN/100 ml within 1 m of a
nearby water course (102 m from the tank)
Nitrate experienced a low rate of reduction and was found at
relatively high concentrations within 1 m of adjacent water course at
one site (max 12.3 mg L-1 NO3-N)
Problem of OSWTS, which may have their subsoil treatment systems
either fully or partially by passed - This warrants further investigation -
Correct identification of the source of contamination is vital
An appropriate inspection system, which is capable of detecting
pollution arising from OSWTS is needed


Conclusions
Of the sites intrusively investigated:
Site D and Site S had inappropriately designed percolation systems
Site F had a suspected broken inflow pipe, concern that a portion of
greywater maybe piped separately
Sites J and K had pipes discharging directly to a watercourse.
The system at Site D had never been switched on (Homeowner,
pers. comm.)

Therefore, there were issues relating to installation and maintenance
even before the permeability and characteristics of the subsoil and soil
were taken into account.
Effective education of homeowners regarding the correct care and
maintenance of their OSWTS is crucial.

Acknowledgements
Patrick Rafferty1, Eddy Minet1, Suzanne Linnane1, Alison Orr2 &
Raymond Flynn2
1 Centre for Environment & Freshwater Studies, DkIT

2 Queens University Belfast

This work was funded by the Western River Basin District, ESB
International and the Department of the Environment, Heritage
and Local Government

McCarthy, V., Flynn, R., Orr, A., Minet, E., Rafferty, P., Meehan, R. and Linnane S. (2010) A field study assessing the impact of onsite wastewater
treatment systems (OSWTS) on surface water in the Milltown Lake catchment, Co. Monaghan .

valerie.mccarthy@dkit.ie www.valeriemccarthy.me

Acknowledgements
The National Federation of Group Water Schemes and the Churchill and
Oram Group Water Scheme
The homeowners for allowing us access their land and OSWTS
Eddy Minet, Michelle Reilly and other staff and students at the Centre
for Freshwater and Environmental Studies, DkIT
Jim O’ Donohue and Tom McGoldrick questionnaires and site
investigation
Robbie Meehan and Nevin Traynor for carrying out site assessments
and percolation tests

valerie.mccarthy@dkit.ie www.valeriemccarthy.me

A field study assessing the impact of on site valerie mc-carthy_slideshare

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