Question
Can a Landscape Architect design detention ponds under the LA seal in the State of Georgia?
Can a Landscape Architect design water quality systems under the LA seal in the State of Georgia?

Answer
Short answer: YES

Additional information:
Here is the GA Licensing board website, to see if your particular LA is one of that State抯 877 licensed professionals:
http://sos.georgia.gov/plb/landscape/

Here is a link to GA State Law governing LA practice in that state (in Adobe Acrobat Format):
http://sos.georgia.gov/cgi-bin/PLBLaws.asp?Board=04

It is only 9 pages long, and includes descriptions of services that can be offered by a licensee.

Some states limit the practice to certain areas, while others allow great latitude.  Notwhithstanding, a person NOT licensed cannot call himself a Landscape Architect OR practice Landscape Architecture (with certain exceptions), without first being licensed by the State of Georgia.

The scope of services allowe by a Landscpae Architect in Georgia are outlined in 43-43-1 (3):

搼Landscape architecture?means the performance of professional services, including,
but not limited to, consultation, investigation, planning, design, preparation of drawings
and specifications, and responsible supervision, all in connection with the preservation
or determination of proper land uses, natural land features, esthetics, planting plans, the
shaping of land to produce the best functional and esthetic effect, and grading plans
with determination of drainage. This term shall also include the consideration of
environmental problems involving land areas, as such problems relate to the public
health, safety, and welfare.?br>
Now, even though a Landscape Architect IS allowed to 搒hape the land?and determine drainage, he/she may not be the best person for the task, if it is a large or intricate drainage solution.  

I personally have designed retention & detention ponds and calculated metered flows, but my liability insurance company was not happy.  They would prefer a Civil Engineer do the 揾eavier?calculations.  

My personal "cut-off" comfort level is no more than 20 acres of tributary area, but your LA may have specialized training in this area of practice.  I worked with several Civil Engineers in my first 15 years of practice, and am comfortable designing these structures.

Best of luck ~Marc