Concerns with chicken paddocks on a septic drain field?

I am new to permaculture, but want to convert my 5 acres of semi-desert in SW Idaho to the permaculture way. Currently building a hugelkultur bed and recently picked up 5 laying hens (Buff Orpington).

Next project is to build them paddocks(they came with a mini coop and a chicken tractor). I want them to be as self-sustaining as possible so am planning on making a 70'X70" paddock system(each paddock would be 35'X35) with the mini-coop in the middle so it won't need to be moved around. This will give each bird close to 1000 square feet(70'X70"=4900 sq ft.)

Land info: Semi-desert-very little rain in the summer(average less than .40 inch of rain per month June-Aug with high temps of about 90 degees, grow zone #6.

Drain field info: Average depth is noted at 3 feet deep. System consists of three 75 foot legs spaced 9 feet apart. Soil noted as silty sand.

My questions are:
Are there parasite or pathogen concerns with raising chickens over a drain field?

Any concerns(compaction or other) with 5 chickens damaging the drain field in any way with the size paddock system mentioned and a weekly rotation of the birds? 

Any estimate of % of feed the birds would get from this paddock system? Vegitation consists mostly of cheat grass and weeds(some weeds are 4 to 5 feet tall).

Reason for wanting it over the drain field are that while the drain field is working correctly and no surface moisture is present, it does sub-irrigate to some extent and manages to stay green over the summer while the rest of the pasture is crispy brown since it does not have irrigation.

I know theoretically you're supposed to be fine since pathogens shouldn't be able to leave the drain field.

The only criteria I can give you which should be foolproof would be whether water naturally pools in that area?

If you ever see water sitting there and it isn't minutes after a downpour, you might have problems since the immediate ground would be saturated.

As long as your drain field was built to code, and there aren't any strange water effects from it, you should be fine, though someone might have had a bad experience they could link to keeping chickens above a drain field.

I found this forum entry that might be interesting for you.

http://www.countrybynet.com/forums/land-care/24679-putting-chicken-coop-over-septic-drain-field.html

Summary: several agreed, several disagreed, the only concensus, don't put in a foundation for your coop, and don't put anything heavier than a person over the drain field.

Nope, never any standing water or even squishiness underfoot on the drain field, unless have had recent rain. Drain field appears to be in good working order.

The mini coop is only 3'X4', so no foundations are in the plan. Actually, instead of putting it in the cener of the system with access to all four paddocks, I have changed my mind to moving it from paddock to paddock to avoid the disadvantages of the Wagon Wheel type setup.   

my drain feild is in the pasture my goats graze over it freely, frankly any dangerous to human microlife would have to survive in the chickens too which is highly unlikely

My (uneducated) guess is that you should be fine.  Most pathogens will not pass from species to species.  As unlikely as it is, even if they were able to eat their way into the roots of your 'pasture', they would not survive into the edible parts of the plants that the chooks are going to eat.

If mankind continues to mess with genetics, (as "we" are doing with GMO's), this info may become obsolete.  Personally, I wouldn't worry about it.  Maybe, my children/grand children will have to worry about it some day.

Somebody, please correct me if I am wrong.

I rotate my paddock frequently with one of spots being over a septic field.

My view is as long as the septic is functioning as intended, there isn't a heightened sense of risk for issues. The hens are essentially on 'lawn'.