Parts per million and milligrams per liter are the same as long as you are working with dilute solutions in water where one liter is one kilogram.
Since the US EPA only considers manganese a secondary contaminamt, I suspect that the rather dire effects described at the first link of my earlier posting probably apply to much higher levels than 0.05 mg/liter.
At the levels that usually occur in water supplies, both iron and manganese are more associated with taste and color, including staining of clothes in laundry. It is also possible to get iron-based bacteria that make a mess of things.
There is a pretty good site that the state of Maine operates that discusses a lot about water quality and wells.
http://www.umaine.edu/WaterResearch/outreach/safe_drinking_water_digest.htm
There is a recent story about manganese is drinking water in Madison, Wisconsin that cites an EPA recommended health standard of 300 ppb, which is 0.3 mg/liter.
http://www.madison.com/archives/read.php?ref=/tct/2006/05/17/0605170487.php
http://www.dhs.ca.gov/ps/ddwem/chemicals/manganese/notificationlevel.htm
The following is copied from the site at the link immediately above:
"However, manganese at very high levels can pose a neurotoxic risk (ATSDR, 2000; US EPA, 1996). For example, neurologic damage (mental and emotional disturbances, as well as difficulty in moving—a syndrome of effects referred to as "manganism") has been reported to be permanent among manganese miners exposed to high levels of airborne manganese for long periods of time.
Lower chronic exposures in the workplace resulted in decrements in certain motor skills, balance and coordination, as well as increased memory loss, anxiety, and sleeplessness (ATSDR, 2000). US EPA (1996), in developing an oral reference dose for manganese based on dietary intake, mentions an epidemiological study in Greece that showed an increase in neurologic effects such as weakness and fatigue, disturbances in gait, and neuromuscular effects, in people whose drinking water contained 1.6 to 2.3 mg/L. Uncertainties about the levels of dietary manganese and the amount of drinking water consumed did not enable US EPA to use these data for risk assessment purposes."
Manganese in your water is in the range where there is a lot of uncertainty regarding health effects if you drink it. If it were mine, I would do more research and probably treat it before drinking it long term. However, it does not appear to be something that requires immediate action to avoid health problems.