Microscopic Particulate Analysis ( MPA ) for Ground Water Under Direct Influence ( GWUDI ) Guide


Microscopic Particulate Analysis (MPA) For Evaluation of Ground Water

I.        What is “a ground water source under direct influence of surface water”?

 The EPA Guidance Manual defines a ground water source under direct influence of surface water as water in which there is either:

“significant occurrence of insects or macroorganisms, algae, organic debris, or large-diameter pathogens such as Giardia lamblia.


“significant and relatively rapid shifts in water characteristics such  as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water condition.”


II.     What information is useful in classification?


Historical water quality records

  1. At least three years of Total Coliform and/or Fecal Coliform
  2. Turbidity and temperature records including those of nearby surface water
  3. No history of a known or suspected outbreak of Giardia, Cryptosporidium or other pathogenic organisms associated with surface water that has been attributed to the
  4. No evidence of particulate matter associated with surface


On site inspection

  1. No evidence for surface water
  2. Sufficient distances from surface water


III.   What is MPA and how can it be used to differentiate ground water under surface influence?


The premise behind the use of microscopic particulate analysis (MPA) is that surface waters are subject to contamination by pathogens such as Giardia and that there are other organisms whose natural habitat is limited to surface waters. If these surface water organisms are found in ground water, then the water is subject to contamination with Giardia cysts and other pathogens.  Indicators of  surface water contamination of ground waters include: Giardia, coccidia including Cryptosporidium, diatoms and certain other algae, rotifers, green plant material, and insect parts.



Cymbella (Diatom)

Cymbella (Diatom)    

 Naviculla (Diatom) 

Naviculla (Diatom)


IV.   How is test conducted?

 Samples are collected in accordance with the EPA “Consensus Method for Determining Groundwaters Under the Direct Influence   of Surface Water Using Microscopic Particulate Analysis (MPA).” Two sampling events are recommended, one during a dry period and a second during a wet period. Turbidity, temperature, rainfall and stream flow records, and conductivity etc. may be used for guidance for when to test.


V.      How are results interpreted?

 When organisms such as Giardia , coccidian, such as Cryptosporidium, insect parts including nymphs, larvae and eggs, rotifers, diatoms and other algae are detected in groundwater, they  are useful as indicators of surface contamination. The process of scoring microscopic results with relative risk factors is beneficial in classifying questionable supplies.


Primary Indicators

Giardia – A protozoan parasite. Occurrence in water sample must be confirmed by identification of two or more morphological characteristics, nuclei, axoneme and/or median body.

Coccidia (Cryptosporidium and other coccidia) – Coccidia are protozoan parasites of vertebrates. Cryptosporidium, a pathogen of concern to human health is small in size (approximately 4-7 µm diameter). Cryptosporidium is very difficult to identify without IFA staining, and its occurrence is confirmed by identification of sporozoites within the oocyst.

Diatoms – Diatoms are prevalent in creeks and streams, and they require sunlight for photosynthesis and continued survival. It is important that the diatoms contain normal internal morphology including pigment in order to be significant for the purposes of this analysis.

Other Algae – Includes several groups of algae including the green and blue-green algae (Cyanobacteria). These algae are abundant in surface water and do not generally persist in the absence of sunlight.

Insects/Larvae – This category includes insects and their larvae and eggs. Insect parts are not as significant as intact organisms since insects molt and the external skeleton could persist in water for long periods of time.

Golenkinia (Green Alga) & Cyclotella (Diatom)   

Golenkinia (Green Alga) & Cyclotella (Diatom)

Coelastrum (Green Alga)

Coelastrum (Green Alga)

Tribonema (Golden Alga)   

Tribonema (Golden Alga)

Anabaena (Blue-Green Alga)  

Anabaena (Blue-Green Alga)

Insect Wing Scale

Insect Wing Scale


Rotifers – Organisms ranging in size 70-500 µm that are medium to good indicators of surface water influence, particularly when supported by the presence of other indicators.

Plant Debris – This category, in our opinion, is significant only when it relates to chlorophyll containing fragments of plant tissue, since the plant debris could persist in water for extended periods of time much beyond the viability of Giardia or other pathogen cysts.


Secondary Indicators

Nematodes – Nematodes and or their eggs are common in surface water and in ground waters with detritus and organic debris.

Crustaceans – Many species occur in surface waters.

Amoebae – Free living amoebae. Large numbers of amoebae in groundwater may indicate substantial bacterial populations or organic detritus in the water.

Non-photosynthetic flagellates and ciliates – Free-living protozoa are extremely common in healthy surface sources. Like amoebae, they feed on bacteria, algae, small metazoans, other protozoa and extraneous debris. Although many flagellates are photosynthetic, there are a number of species that grow in the absence of light providing sufficient dissolved nutrients are available.

Photosynthetic flagellates – Includes species such as Euglena. While these organisms are photosynthetic, many can persist in the dark for months and because they are motile, their presence may not be indicative of surface water contamination.

Other: Other organisms frequently seen in MPA samples include the iron bacteria. The presence of iron bacteria does not have significance for surface infiltration, but large numbers of iron bacteria can produce biofouling of the well.

The EPA risk factor tables are used to weight the results of MPA analyses. The greatest weight is given to the primary indicators.


Table 1 assigns a rating (Not Significant through Extremely Heavy).


Table 2 uses the ratings assigned by Table 1 to determine the Relative  Risk Factor.

Table 1: Numerical range of each primary bio-indicator (particulate) counted per 100 gallons water.

Indicators of surface water1 EH3 H M R NS
Giardia2 >30 16 to 30 6 to 15 1 to 5 <1
Coccidia2 >30 16 to 30 6 to 15 1 to 5 <1
Diatoms4 >150 41 to 149 11 to 40 1 to 10 <1
Other Algae4 >300 96 to 299 21 to 95 1 to 20 <1
Insects/Larvae >100 31 to 99 16 to 30 1 to 15 <1
Rotifers >150 61 to 149 21 to 60 1 to 20 <1
Plant Debris4 >200 71 to 200 26 to 70 1 to 25 <1


  1. According to EPA “Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources”, March, 1991
  2. If Giardia cysts or coccidian are found in any sample, irrespective of volume, score as
  3. Key: EH-Extremely High, H-Heavy, M-Moderate, R-Rare, NS-Not Significant
  4. Chlorophyll Containing


Table 2: Relative surface water risk factors associated with scoring or primary bio-indicators (particulate) present during MPA of subsurface water sources.

Indicators of surface water1 Relative Risk Factor3
Giardia 40 30 25 20 0
Coccidia 35 30 25 20 0
Diatoms 16 13 11 6 0
Other Algae 14 12 9 4 0
Insects/Larvae 9 7 5 3 0
Rotifers 4 3 2 1 0
Plant Debris 3 2 1 0 0


  1. According to EPA “Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources”, March, 1991
  2. Key: EH – Extremely High, H – Heavy, M – Moderate, R – Rare, NS – Not Significant
  3. Risk of surface water contamination:

>20 – High risk, 10-19 – Moderate risk, <9 – Low Risk


*Note: Any finding of Giardia or Cryptosporidium results in a minimum risk factor of 20.


*Appendix 1. Tables 1 & 2 from Vasconcelos, J. 1992 Consensus Method for Determining Groundwaters Under the Direct Influence of Surface Water Using Microscopic Particulates Analysis (MPA). U.S. Environmental Protection Agency, Region 10 Oct. p. 30-31.